Invited papers -- Two sided matching markets with strongly correlated random preferences -- Communicating Finite state machines and an extensible toolchain for multiparty session types -- Contributed papers -- First-Order logic and its Infinitary Quantifier Extensions over Countable Words -- From Symmetry to Asymmetry: Generalizing TSP Approximations by Parametrization -- A Poly-Log Competitive Posted-Price Algorithm for Online Metrical Matching on a Spider -- Computational Complexity of Covering Disconnected Multigraphs -- The Complexity of Bicriteria Tree-depth -- TS-Reconfiguration of Dominating Sets in circle and circular-arc graphs -- Bipartite 3-Regular Counting Problems with Mixed Signs -- The Satisfiability Problem for a Quantitative Fragment of PCTL -- Beyond the BEST Theorem: Fast Assessment of Eulerian Trails -- Regular Model Checking with Regular Relations -- Minimum Consistent Subset of Trees -- Parameterized Complexity of Finding Subgraphs with Hereditary Properties onHereditary Graph Classes -- The Space Complexity of Sum Labelling -- On Minimizing Regular Expressions Without Kleene Star -- Computational Complexity of Computing a Quasi-Proper Equilibrium -- Computational Complexity of Synchronization under Sparse Regular Constraints -- On Dasgupta's hierarchical clustering objective and its relation to other graph parameter -- Mengerian Temporal Graphs Revisited -- Faster FPT Algorithms for Deletion to Pairs of Graph Classes -- Fast Algorithms for the Rooted Triplet Distance Between Caterpillars -- Deciding Top-Down Determinism of Regular Tree Languages -- Propositional Gossip Protocols -- Complexity of word problems for HNN-extensions -- On Finding Separators in Temporal Split and Permutation Graphs -- The Possible Winner Problem with Uncertain Weights Revisited -- Streaming Deletion Problems Parameterized by Vertex Cover -- On the Hardness of the Determinant: Sum of Regular Set-Multilinear Circuits -- Concentration of Collision Estimator -- Valency-based Consensus under Message Adversaries without Limit-Closure. .

This book constitutes the proceedings of the 23rd International Symposium on Fundamentals of Computation Theory, FCT 2021, held in Athens, Greece, in September 2021. The 30 full papers included in this volume were carefully reviewed and selected from 94 submissions. In addition, the book contains 2 invited talks. The papers cover topics of all aspects of theoretical computer science, in particular algorithms, complexity, formal and logical methods.