This book constitutes the refereed proceedings of the 6th International Symposium on NASA Formal Methods, NFM 2014, held in Houston, TX, USA, April 29 - May 1, 2014. The 20 revised regular papers presented together with 9 short papers were carefully reviewed and selected from 107 submissions. The topics include model checking, theorem proving, static analysis, model-based development, runtime monitoring, formal approaches to fault tolerance, applications of formal methods to aerospace systems, formal analysis of cyber-physical systems, including hybrid and embedded systems, formal methods in...