This book constitutes the refereed proceedings of the 21st International Colloquium on Theoretical Aspects of Computing, ICTAC 2024, held in Bangkok, Thailand, during November 25–29, 2024. The 21 full papers presented here were carefully reviewed and selected from 46 submissions.

Automata, Languages, and Learning.- Well Behaved (Co)algebraic Semantics of Regular Expressions in Dafny.- Jump Complexity of Deterministic Finite Automata with Translucent Letters.- Learning Closed Signal Flow Graphs.- Dual Adjunction Between $Omega$ Automata and Wilke Algebra Quotients.- On Concurrent Program Algebra and Demonic Automata.- Bisimulations and Logics for Higher Dimensional Automata.- Algorithms and Complexity.- Maximizing Weighted Dominance in the Plane.- Generalized Parikh Matrices For Tracking Subsequence Occurrences.- Card Based Protocols with Single Card Encoding.- Graphs and Games.- Winning Strategy Templates for Stochastic Parity Games towards Permissive and Resilient Control.- Disconnection Rules are Complete for Chemical Reactions.- Verification with Common Knowledge of Rationality for Graph Games.- Process Calculi.- Reversibility in Process Calculi with Nondeterminism and Probabilities.- A Theory of Proc Omata and a Proof Technique for Parameterized Process Architectures.- Formal Foundations for Efficient Simulation of MOM Systems The Refinement Calculus for Object Oriented Event Graphs.- Verification and Reasoning.- Verifying Type Safety for Isabelle/Solidity.- History Based Reasoning about Behavioral Subtyping.- Switched Systems in Coq for Modeling Periodic Controllers.-  A quantum inspired mechanical method for proving of Ramsey’s theorem by symbolic computation over the finite field GF(2).- Runtime Enforcement with Event Reordering.