Fundamental Approaches to Software Engineering : 23rd International Conference FASE 2020 Held as Part of the European Joint Conferences on Theory and Practice of Software ETAPS 2020 Dublin Ireland April 25–30 2020 Proceedings Requirements engineering Software architectures Specification Software quality Validation Verification of functional and non functional properties Model driven development and model transformation Software processes Security and software evolution. This open access book constitutes the proceedings of the 23rd International Conference on Fundamental Approaches to Software Engineering FASE 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 23 full papers 1 tool paper and 6 testing competition papers presented in this volume were carefully reviewed and selected from 81 submissions. The papers cover topics such as requirements engineering software architectures specification software quality validation verification of functional and non functional properties model driven development and model transformation software processes security and software evolution.