Foundations of Software Science and Computation Structures : 23rd International Conference FOSSACS 2020 Held as Part of the European Joint Conferences on Theory and Practice of Software ETAPS 2020 Dublin Ireland April 25–30 2020 Proceedings This open access book constitutes the proceedings of the 23rd International Conference on Foundations of Software Science and Computational Structures FOSSACS 2020 which took place in Dublin Ireland in April 2020 and was held as Part of the European Joint Conferences on Theory and Practice of Software ETAPS 2020. The 31 regular papers presented in this volume were carefully reviewed and selected from 98 submissions. The papers cover topics such as categorical models and logics; language theory automata and games; modal spatial and temporal logics; type theory and proof theory; concurrency theory and process calculi; rewriting theory; semantics of programming languages; program analysis correctness transformation and verification; logics of programming; software specification and refinement; models of concurrent reactive stochastic distributed hybrid and mobile systems; emerging models of computation; logical aspects of computational complexity; models of software security; and logical foundations of data bases.