This book constitutes the proceedings of the 39th International Conference on Computer Safety, Reliability and Security, SAFECOMP 2020, held in Lisbon, Portugal, in September 2020.*The 27 full and 2 short papers included in this volume were carefully reviewed and selected from 116 submissions. They were organized in topical sections named: safety cases and argumentation; formal verification and analysis; security modelling and methods; assurance of learning-enabled systems; practical experience and tools; threat analysis and risk mitigation; cyber-physical systems security; and fault injection and fault tolerance.*The conference was held virtually due to the COVID-19 pandemic.The chapter Assurance Argument Elements for Off-the-Shelf, Complex Computational Hardware is available open access under an Open Government License 3.0 via link.springer.com.