This book constitutes the refereed proceedings of the 20th International Conference on Verification, Model Checking, and Abstract Interpretation, VMCAI 2019, held in Cascais, Portugal, in January 2019. The 27 full papers presented together with the abstracts of 3 invited keynote talks were carefully reviewed and selected from 62 submissions. VMCAI provides topics including: program verification, model checking, abstract interpretation, program synthesis, static analysis, type systems, deductive methods, program certification, decision procedures, theorem proving, program certification, debugging techniques, program transformation, optimization, and hybrid and cyber-physical systems.