The two open access volumes LNCS 14574 and 14575 constitute the proceedings of the 27th International Conference on Foundations of Software Science and Computation Structures, FOSSACS 2024, which took place in Luxembourg in April 2024. The 24 full papers included in this book were carefully reviewed and selected from 79 submissions. They were organized in topical sections as follows:  Part I: Infinite games; categorical semantics; automata and synthesis;  Part II: Types and programming languages; logic and proofs; infinite-state systems.