This book constitutes the proceedings of the 13th International Symposium on NASA Formal Methods, NFM 2021, held virtually in May 2021.The 21 full and 3 short papers presented in this volume were carefully reviewed and selected from 66 submissions. The papers aim to identify challenges and provide solutions to achieve assurance in mission-critical and safety-critical systems. Examples of such systems include advanced separation assurance algorithms for aircraft, next-generation air transportation, autonomous rendezvous and docking of spacecraft, on-board software for unmanned aerial systems (UAS), UAS traffic management, autonomous robots, and systems for fault detection, diagnosis, and prognostics.

This book constitutes the proceedings of the 14th International Symposium on NASA Formal Methods, NFM 2022, held in Pasadena, USA, during May 24-27, 2022. The 33 full and 6 short papers presented in this volume were carefully reviewed and selected from 118submissions. The volume also contains 6 invited papers. The papers deal with advances in formal methods, formal methods techniques, and formal methods in practice. The focus on topics such as interactive and automated theorem proving; SMT and SAT solving; model checking; use of machine learning and probabilistic reasoning in formal methods; formal methods and graphical modeling languages such as SysML or UML; usability of formal method tools and application in industry, etc.