This book constitutes the refereed proceedings of the 18th International Conference on Software Engineering and Formal Methods, SEFM 2020, held in Amsterdam, The Netherlands, in September 2020. The 16 full papers presented together with 1 keynote talk and an abstract of a keynote talk were carefully reviewed and selected from 58 submissions. The papers cover a large variety of topics, including testing, formal verification, program analysis, runtime verification, meta-programming and software development and evolution. The papers address a wide range of systems, such as IoT systems, human-robot interaction in healthcare scenarios, navigation of maritime autonomous systems, and operating systems. The Chapters "Multi-Purpose Syntax Definition with SDF3", ¿FRed: Conditional Model Checking