The papers are selected by an awards committee that is appointed by the LICS General Chair and consists of between three to five members. The committee is renewed every year; at the discretion of the General Chair, members may be reappointed to the awards committee.
In selecting these papers, the Awards Committee should consider the influence that the papers have had since publication; because of the foundational nature of LICS work, impact is often not fully felt immediately, hence the 20year perspective.
The Award Committee is expected to select 12 papers. However, the Award Committee may choose to select no paper from a given year, or may select up to 3 papers.
All papers from the given year are eligible for this award, except those that are authored or coauthored by members of the Awards Committee. There is no formal nomination process for this award, but input from the LICS community is welcome.
List of all TestofTime Award winners

LICS TestofTime Award Winner in 2021
All papers from LICS 2001 were considered for the 2001 LICS TestofTime Award. The Award Committee consisted of Parosh Abdulla, Manuel Bodirsky (chair), Erich Graedel, and Radha Jagadeesan.
Aaron Stump Clark W. Barrett David L. Dill Jeremy R. Levitt
A Decision Procedure for an Extensional Theory of ArraysMore Information...This is the first paper on a dedicated decision procedure for the theory of arrays, one of the most important theories in applications of Satisfiability Modulo Theories (SMT), particularly in combination with bitvectors for bitprecise reasoning. These applications include symbolicexecution (e.g., SAGE) and boundedmodel checking of software and hardware (CBMC). Thus it was highly influential and formed the basis for other high impact papers in SMT.
Verification of Program termination is of significant practical importance. In this paper, Xi describes a dependent type system that enables the programmer to supply metrics for verifying program termination, in order to establish the guarantee that every welltyped program in this type system is terminating. The type system of the paper supports a general form of recursion, higherorder functions, algebraic datatypes, and polymorphism. Thus, it advances the hope of incorporating the rich body of literature on proving termination into a realistic functional programming language. The promise of this paper has been realised in subsequent research in various research groups. It has also impacted publicly available tools such as LiquidHaskell, a library for general purpose programs in Haskell.
The presentation of the award took place at LICS 2021.

LICS TestofTime Award Winner in 2020
All papers from LICS 2000 were considered for the 2000 LICS TestofTime Award. The Award Committee consisted of Thierry Coquand (chair), PierreLouis Curien, Marcelo Fiore, and Orna Kupferman.
The setting of two player games on finite graphs, used to model interactions between systems and their environments, has been extensively studied in the context of synthesis and control. A key issue for concurrentgames is that it requires probabilistic considerations. This paper handled concurrent games with omegaregular objectives (that is, the winning condition refers to the infinite path that the players generate). It studied the theoretical properties of the games and provided clever algorithms for computing the sets of winning states for three different qualitative modes of winning, reflecting different probabilistic guarantees. By settling several fundamental results of this theory, this is a landmark paper that has been extended to include further qualitative parameters (energy and other weighted games), richer winning conditions, and applications beyond traditional synthesis and control.
This paper brought new light to the study of equirecursive types in the context of the propositionsastypes paradigm; especially to arbitrary recursive types, such as the famous equation (D=D>D) underlying the semantics of the untyped lambdacalculus, as well as selfreferential types featuring in objectoriented programming. Roughly, the key idea is to havea stratified view of such types reflecting more and more precise properties that correspond to iterations of an associated induction step. The paper puts forward this approach and, importantly, formalizes it by means of a very natural modality that shifts the stratification. The resulting system allows akind of guarded approach to general recursion that can avoid the pitfalls of nontermination getting in the way of formal reasoning. The work connected recursion with the methods of Kripke and realizability semantics, belongingto the realm of intuitionistic logic, and, yet to be further explored, to provability logic. These ideas and techniques are now widespread and popular in stepindexed logical relations and in various modal extensions of type theory that are under active investigation.
The presentation of the award took place at LICS 2020.

LICS TestofTime Award Winner in 2019
All papers from LICS 1999 were considered for the 2019 LICS TestofTime Award. The Award Committee consisted of Thomas Ehrhard, Frank Pfenning (chair), and Davide Sangiorgi. Variable binding operators are pervasive in the study of programming languages and logics. Following an idea of Church (1940) these can be reduced to a single binding operator whose syntactic properties have been understood for some time. These two seminal papers propose contrasting approaches to achieve a matching semantic understanding for abstract syntax with binding.
Marcelo P. Fiore Gordon D. Plotkin Daniele Turi
Abstract Syntax and Variable BindingMore Information...Fiore, Plotkin, and Turi instead take a categorical approach, devising binding algebrasthat arepresheaves endowed with both an algebra structure and a substitution structure that are compatible with each other. Abstract syntax with binding is then an initial model. The compatibility of this idea with de Bruijn indices, already popular in formal developments before 1999, has led to many applications of their results in further theory and implementations.
Murdoch Gabbay Andrew M. Pitts
A New Approach to Abstract Syntax Involving BindersMore Information...Gabbay and Pitts employ the permutation model of set theory with atoms due to Fraenkel and Mostowski to represent name abstraction and fresh name generation. This led to a wealth of research using socalled nominal techniques, both in theory and in practical implementations in theorem provers. An extended andrevised version of their paper was published in Formal Aspects of Computing in 2002.
The presentation of the award took place at LICS 2019.

LICS TestofTime Award Winner in 2018
All papers from LICS 1998 were considered for the 2018 LICS TestofTime Award. The Award Committee consisted of Christel Baier (chair), Luca Aceto, Anca Muscholl, and Vaughan Pratt.
Martín Abadi Cédric Fournet Georges Gonthier
Secure Implementation of Channel AbstractionsMore Information...A secure channel is an important primitive in distributed systems programming. It protects the confidentiality of messages while they traverse untrusted networks. It is folklore that a secure channel can be implemented using cryptography, specifically using encryption. However, it is the LICS 1998 paper “Secure Implementation of Channel Abstractions” that formalizes this folklore idea and shows howto mathematically specify that a secure channel has been implemented correctly. In doing so, thepaper also demonstrates, for the first time, how the classic concept of full abstraction of language translations can be used to formalize the preservation of security properties, an idea that has been used several times subsequently.
Samson Abramsky Kohei Honda Guy McCusker
A Fully Abstract Game Semantics for General ReferencesMore Information...This paper presented the first games model of a programming language with higherorder store la ML references. The result came as a big surprise within the communityat the time and was a technical tour de force. Seen from today’s perspective, thearticle was certainly one of the main achievements of game semantics, and extendedthe reach of games models well beyond the work on PCF by Nickau, Hyland and Ong,and Abramsky, Jagadeesan and Malacaria (which received the 2017 Alonzo Church Award). The paper also opened a new field of research, which is still being exploredand exploited today within the gamesemantics community.
The presentation of the award took place at LICS 2018.

LICS TestofTime Award Winner in 2017
All papers from LICS 1997 were considered for the 2017 LICS TestofTime Award. The Award Committee consisted of Amy Felty (chair), Christel Baier, Andrew Pitts, and Nicole Schweikardt.
Richard Blute Josée Desharnais Abbas Edalat Prakash Panangaden
Bisimulation for Labelled Markov ProcessesMore Information...The paper introduces labelled Markov processes as a continuous space variant of deterministic labeled transition systems where the dynamics of the stateaction pairs is given by Markov kernels specifying the probability for measurable sets of successor states. The presented notion of bisimulation of labelled Markov processes is a conservative extension of Larsen and Skou’s bisimulation for discrete probabilistic transition systems. The paper presents a highly nontrivial proof for the transitivity of bisimulation on labelled Markov processes and first steps towards a logical characterization of bisimulation in terms of a probabilistic HennessyMilner logic. By introducing labeled Markov processes and a notion of bisimulation for them, the authors provided important foundations for the formal semantics and analysis of stochastic systems where physical components interact with discrete ones. The paper opened a new research area on continuousspace stochastic models and inspired many researchers to study further properties of labelled Markov processes and variants thereof.
This paper introduced a new and mathematically elegant way of relating the syntax and semantics of programs, using the existing categorytheoretic notion of a distributive law between monads and comonads. Specifically, it gives an abstract view of the structural operational semantics of concurrent processes as distributing behaviour over syntax, one which guarantees the existence of a most abstract, compositional semantics of the language. The paper was an early example of the usefulness of coalgebraic techniques in semantics and has been, and still is, an extremely influential paper within the coalgebra community.
The presentation of the award took place at LICS 2017.

LICS TestofTime Award Winner in 2016
All papers from LICS 1996 were considered for the 2016 LICS TestofTime Award. The Award Committee consisted of Amy Felty, JeanPierre Jouannaud (chair), Naoki Kobayashi, and Glynn Winskel.
Parosh A. Abdulla Karlis Cerans Bengt Jonsson YihKuen Tsay
General decidability theorems for infinitestate systemsMore Information...The LICS1996 paper "General decidability theorems for infinitestate systems" authored by Abdulla, Cerans, Jonsson and Tsay showed that various verification problems are decidable for wellstructured transition systems (WSTS). WSTS is a general class of infinite systems that subsumes a number of important computation models such as Petri nets and lossy channel systems. While the notion of WSTS was introduced in 1990 by Finkel with a slightly restricted format, the present paper proved the decidability of the coverability problem for the general class introduced there by describing a backward algorithm. This powerful technique has been quite influential in the field, and cited ever since as a standard reference on WSTS.
The Linear Logical Framework (LLF) presented in the LICS 1996 paper "A Linear Logic Framework" authored by Cervesato and Pfenning was one of the first logical frameworks to build the notion of linearity directly into the framework, and the first one to do so using a dependentlytyped lambdacalculus in the style of Harper, Honsell, and Plotkin's Logical Framework (LF). As a result, the metatheory of a variety of logical formalisms such as imperative programming languages can be elegantly formalized, with concepts such as program state more directly and concisely encoded using the linear features. The impact of this work continues as newer application areas such as quantum programming languages benefit from the availability of this kind of framework. In addition, this work continues to be influential in the development of new frameworks, such as those with richer dependentlytyped calculi, which build on the foundations established in this paper.
The presentation of the award took place at LICS 2016.

LICS TestofTime Award Winner in 2015
All papers from LICS 1995 were considered for the 2015 LICS TestofTime Award. The Award Committee consisted of Martin Grohe, Dexter Kozen, Dale Miller (Chair) and Prasad Sistla.
Igor Walukiewicz
Completeness of Kozen's Axiomatisation of the Propositional MuCalculusMore Information...The propositional mucalculus (now more commonly known as the modal mucalculus) was first published in 1983 by D. Kozen, although the system had been studied much earlier by J. de Bakker and D. Scott (unpublished notes, 1969). The system subsumes many popular propositional logics of programs, including linear temporal logic, computation tree logic CTL*, and propositional dynamic logic. Kozen proposed a deductive system whose chief rule of inference was a variant of the fixpoint induction rule of D. Park and conjectured it to be complete. During the decade after its introduction, this calculus attracted a great deal of attention within computer science circles. Despite the attention given to this calculus, it was not until this paper by Walukiewicz a dozen years later that the proof system was finally shown to be complete. In order to prove the completeness theorem, several technical innovations were developed. A full version of this paper appeared in Information and Computation in 2000.
The presentation of the award took place at LICS 2015.

LICS TestofTime Award Winner in 2014
All papers from LICS 1994 were considered for the 2014 LICS TestofTime Award. The Award Committee consisted of Thierry Coquand, Dexter Kozen (chair), Leonid Libkin and Frank Pfenning. Two awards were made.
Martin Hofmann Thomas Streicher
The groupoid model refutes uniqueness of identity proofsMore Information...This paper opened a series of investigations leading to the development of homotopy type theory, an area of intense current research that touches on the very foundations of the field. The paper contained two key insights: first, that it follows from the rules of type theory that types have a groupoid structure; and second, that groupoids form a model of type theory. The results were quite surprising at the time and opened up a new front in the quest to understand the tradeoffs between intensional and extensional type theory. Although later revised and extended by the authors and others, the original insights were in the original LICS paper and were an essential forerunner of more recent develop ments on univalent foundations that led to homotopy type theory in its present form.
JeanYves Girard had introduced linear logic about seven years prior to the appearance of this paper and, in several subsequent papers, cemented its role as a quintessential, unifying logic. In this 1994 LICS paper, Miller showed that linear logic could also be used as a logical framework, providing means for the elegant, highlevel representation of formal systems. Miller gives examples from the domains of logic (natural deduction) and programming languages (opera tional semantics, including effects). This appropriated linear logic for purposes of practical specifications of stateful and concurrent systems. This work has been quite influential over the years and even up to the present with the cur rently active area of behavioral types.
The presentation of the awards took place at CSLLICS 2014.

LICS TestofTime Award Winner in 2013
All papers from LICS 1993 were considered for the 2013 LICS TestofTime Award. The Award Committee consisted of JeanPierre Jouannaud, Martin Grohe, Tom Henzinger, and Prakash Panangaden (chair). Three papers have been selected.
This paper introduces a general framework for solving set constraints which were introduced earlier in a logic programming setting by Jaffer and Heintze. While others had used specialized tools to attack this problem the approach of this paper remains fundamental shedding light on the significance of the monadic class. They showed that the satisfiability problem for set con straints is complete for NEXPTIME and obtained many decidability and complexity results for extensions of set constraints. Though there have been many extensions this paper remains the reference on the topic.
Bisimulation had been for about 20 years a central concept in concurrency theory when this paper appeared. However, there were numerous variations and adaptations for different types of systems. This paper presented a very abstract and general definition that applied across a large class of systems and which easily specializes to many cases such as Milner’s strong bisim ulation, historypreserving bisimulation. This paper was the precursor to many later papers on presheaf models for concurrency. This was expressed in the original paper as a promising line of future research and was spectacularly vindicated in subsequent years largely through the work of Winskel and his collaborators. It even served as the inspiration for future work on probabilistic bisimulation though that was not realized at the time.
This paper marks the beginning of a very successful fusion of ideas from type theory and concurrency. Milner’s πcalculus from the late 1980s was a pioneering example of a mobile calculus and initiated a new direction of research in concurrency theory. Milner’s original type discipline for this calculus was rudimentary. Pierce and Sangiorgi introduced a more refined type system that distinguishes between different ways in which a process can access a channel. This leads to a natural subtype relation between processes and to stronger theorems about the encodings of the λcalculus into the πcalculus. This highly cited paper is the inspiration for many later typed mobile calculi.
The presentation of the awards will take place at LICS 2013.

LICS TestofTime Award Winner in 2012
For the 2012 LICS TestofTime Award, all papers from LICS 1992 were considered. The Award Committee consisted of Martin Grohe, Prakash Panangaden, Andre Scedrov (Chair), and Ashish Tiwari. Two papers have been selected.
Thomas A. Henzinger Xavier Nicollin Joseph Sifakis Sergio Yovine
Symbolic model checking for realtime systemsMore Information...The paper made a crucial contribution to the success of symbolic model checking for analyzing timed and hybrid systems. The paper also led to the development of the Kronos tool, which was also very influential.
The paper was very influential in the programming language community. It paved the way for thinking of all kinds of new phenomena within the typechecking framework.
The presentation of the awards took place on June 27, 2012 during the Award Presentations session at LICS 2012.

LICS TestofTime Award Winner in 2011
For the 2011 LICS TestofTime Award, all papers from LICS 1991 were considered. The Award Committee consisted of Radha Jagadeesan, Tom Henzinger, Catuscia Palamidessi, and Andy Pitts (Chair). Three papers have been selected.
This is one of the papers behind "partialorder methods" in model checking, which is an important approach for fighting state explosion in the analysis of asynchronous systems. The paper established the theory for proving the correctness of partialorder algorithms for checking general temporal properties. This line of research was highly influential and affects the way explicitstate model checkers are being built today.
Joshua S. Hodas Dale A. Miller
Logic programming in a fragment of intuitionistic linear logicMore Information...This is one of the first explorations of logic programming style proof search in Girard's linear logic. The paper radically changed the perception of what logic programming might be, on the heels of linear logic changing the perception of what logic might be. In contrast to the traditional intuitionistic basis of logic programming, this new foundation permitted "stateful" (eg. modelling database updates) and "resourceful" (eg. in linguistic models) declarative models.
Dexter C. Kozen
A completeness theorem for Kleene algebras and the algebra of regular events More Information...This paper solved an important open problem to do with the axiomatization of the algebra of regular events. Although other solutions were found independently by Krob and BloomEsik, what made Kozen's solution in terms of conditional equations so influential is its elegant simplicity and generality. Kozen went on to develop from it a beautiful theory of Kleene algebras, with applications to various aspects of programming theory and logic.
The presentation of the awards took place on June 22, 2011 during the Award Presentations session at LICS 2011.

LICS TestofTime Award Winner in 2010
For the 2010 LICS TestofTime Award, all papers from LICS 1990 were considered. The Awards Committee consisted of Glynn Winskel (chair), JeanPierre Jouannaud and John Mitchell. In view of the weight of highlyinfluential papers, across a range of areas, the committee has taken the exceptional step of selecting four papers. They are:
Rajeev Alur Costas Courcoubetis David L. Dill
Modelchecking for realtime systemsMore Information...Brief Citation: This paper was a pioneer in the model checking of realtime systems. It provided a polynomialspace algorithm for the model checking of a realtime logic (an extension of CTL with timing constraints) with respect to a continuoustime model. Its techniques are still used extensively and results of this paper form part of almost any course or tutorial on realtime verification.
Jerry R. Burch Edmund M. Clarke Kenneth L. McMillan David L. Dill L. James Hwang
Symbolic model checking: 10^20 states and beyondMore Information...Brief Citation: This paper revolutionized model checking. Through its symbolic representation of the state space using Randy Bryant's Binary Decision Diagrams (BDDs) and its careful analysis of several forms of model checking problems, backed up by empirical results, it provided a first convincing attack on the verificationn of largestate systems. The paper was a major agent in establishing BDDs as a tool in mainstream computer science.
Brief Citation: This paper asked what has proved to be a very important question, whether the firstorder theory of onestep rewriting is decidable. The paper settled the question positively for the theory of ground rewrite systems using innovative techniques on tree automata. Its techniques rekindled an interest in automata theory on finite trees, now a major topic, with many current applications from rewriting through to security, program analysis and concurrency.
Brief Citation: This paper showed what was really going on with the classic method of solving domain equations. By separating positive and negative occurrences of the unknown in a domain equation, it gave an elegant categorytheoretic treatment of recursively defined domains that extends the wellunderstood and widelyused methods of initialalgebra semantics. Its methods are now standard. They led to new techniques for relating operational and denotational semantics, and new mixed induction/coinduction principles.
The presentation of the awards took place on July 12th, 2010 at the Business Meeting of LICS 2010.

LICS TestofTime Award Winner in 2009
For the 2009 LICS TestofTime Award, all papers from LICS 1989 were considered. The Awards Committee consisted of Rohit Parikh (chair), Phokion Kolaitis, and Glynn Winskel. The Committee has selected the following paper for the LICS TestofTime Award:
Brief Citation: Eugenio Moggi's LICS'89 paper changed the way we think about programs. According to the paper a program denotes a morphism from values to computations, and the nature of computations is determined by a monad. A 'computational lambda calculus' is built around this idea and shown to be sound and complete with respect to a categorical semantics. But more important than this technical result is the great unifying and systematizing power the ideas of the paper give to the semantics of computation. The paper has had a widespread influence on the theory of programming languages; monads are now a standard tool in modifying types.
The presentation of the award took place on August 12, 2009 during the Award Presentations session at LICS 2009.

LICS TestofTime Award Winner in 2008
For the 2008 LICS TestofTime Award, all papers from LICS 1988 were considered. The Awards Committee consisted of Rajeev Alur (Chair), Samson Abramsky, and Dexter Kozen. The Committee has selected the following paper for the LICS TestofTime Award:
Brief Citation: A central problem in formal approaches to system design and verification concerns establishing a refinement relationship between two descriptions of the system with different levels of detail. This paper shows that the method based on refinement maps is a complete proof technique provided the lowerlevel description is enriched with history and prophecy variables. The paper contains a conceptually clean solution to a practical verification problem, and the resulting ideas continue to provide guidance for designing specification languages as well as for formalizing correctness proofs of complex designs even today.
The presentation of the award took place on June 25, 2008 during the Award Presentations session at LICS 2008.

LICS TestofTime Award Winner in 2007
For the 2007 LICS TestofTime Award, all papers from LICS 1987 were considered. The Awards Committee consisted of Yuri Gurevich (Chair), Rajeev Alur, and Glynn Winskel. The Committee has selected the following two papers for the LICS TestofTime Award; the papers are listed in the order they appeared in the LICS 1987 proceedings:
The presentation of the awards took place on July 10, 2007 during the Awards Ceremony session at LICS 2007.

LICS TestofTime Award Winner in 2006
For the 2006 LICS TestofTime Award, all papers from LICS 1986 (the first LICS) were considered. The Awards Committee consisted of Samson Abramsky (Chair), Rajeev Alur, and Yuri Gurevich. The Committee has selected the following three papers for the LICS TestofTime Award; the papers are listed in the order they appeared in the LICS 1986 proceedings:
E. Allen Emerson ChinLaung Lei
Efficient Model Checking in Fragments of the Propositional MuCalculus (Extended Abstract)More Information...Moshe Y. Vardi Pierre Wolper
An AutomataTheoretic Approach to Automatic Program Verification (Preliminary Report)More Information...The presentation of the awards took place on August 12, 2006 during the 2006 LICS Business Meeting.