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.

Learning and Inference.- Should We Learn Probabilistic Models for Model Checking? A New Approach and an Empirical Study.- Bordeaux: a tool for thinking outside the box.- Test Selection.- Bucketing Failing Tests via Symbolic Analysis.- Selective Bisection Debugging. - On the Effectiveness of Bug Predictors with Procedural Systems: a Quantitative Study.- Program and System Analysis.- Inference and Evolution of TypeScript Declaration Files.- Explicit Connection Actions in Multiparty Session Types.- Change and Delay Contracts for Hybrid System Component Verification. - Precise Version Control of Trees with Line-based Version Control Systems.- Graph Modelling and Transformation.- StaticGen: Static Generation of UML Sequence Diagrams.- Inter-Model Consistency Checking using Triple Graph Grammars and Linear Optimization Techniques.- GTS Families for the Flexible Composition of Graph Transformation Systems.- Symbolic Model Generation for Graph Properties. - Model Transformations.- TraceabilityMappings as a Fundamental Instrument in Model Transformations.- Reusing Model Transformations through Typing Requirement Models.- Change-preserving model repair. -A deductive approach for fault localization in ATL model transformations. -Configuration and Synthesis.- OpenSAW: Open Security Analysis Workbench.- Visual Configuration of Mobile Privacy Policies.- Automated Workarounds from Java Program Specifications based on SAT Solving. -Slicing from Formal Sematics: Chisel.- EasyInterface: A toolkit for rapid development of GUIs for research prototype tools.- Software Product Lines.- Family-Based Model Checking with mCRL2.- Variability-specific Abstraction Refinement for Family-based Model Checking.- A Unified and Formal Programming Model for Deltas and Traits.