This book constitutes the proceedings of the 20th International Conference on Fundamental Approaches to Software Engineering, FASE 2017, which took place in Uppsala, Sweden in April 2017, held as Part of the European Joint Conferences on Theory and Practice of Software, ETAPS 2017. The 23 papers presented in this volume were carefully reviewed and selected from 91 submissions. They were organized in topical sections named: learning and inference; test selection; program and system analysis; graph modeling and transformation; model transformations; configuration and synthesis; and software product lines.