Since the writing of NASA/SP-6105 in 1995, systems engineering at the National Aeronautics and Space Administration (NASA), within national and international standard bodies, and as a discipline has undergone rapid evolution. Changes include implementing standards in the International Organization for Standardization (ISO) 9000, the use of Carnegie Mellon Software Engineering Institute's Capability Maturity Model(R) Integration (CMMI(R)) to improve development and delivery of products, and the impacts of mission failures. Lessons learned on systems engineering were documented in reports such as those by the NASA Integrated Action Team (NIAT), the Columbia Accident Investigation Board (CAIB), and the follow-on Diaz Report. Out of these efforts came the NASA Office of the Chief Engineer (OCE) initiative to improve the overall Agency systems engineering infrastructure and capability for the efficient and effective engineering of NASA systems, to produce quality products, and to achieve mission success. In addition, Agency policy and requirements for systems engineering have been established. This handbook update is a part of the OCE-sponsored Agency wide systems engineering initiative. In 1995, SP-6105 was initially published to bring the fundamental concepts and techniques of systems engineering to NASA personnel in a way that recognizes the nature of NASA systems and the NASA environment. This revision of SP-6105 maintains that original philosophy while updating the Agency's systems engineering body of knowledge, providing guidance for insight into current best Agency practices, and aligning the handbook with the new Agency systems engineering policy. The update of this handbook was twofold: a top-down compatibility with higher level Agency policy and a bottom-up infusion of guidance from the NASA practitioners in the field. The approach provided the opportunity to obtain best practices from across NASA and bridge the information to the established NASA systems engineering process. The attempt is to communicate principles of good practice as well as alternative approaches rather than specify a particular way to accomplish a task. The result embodied in this handbook is a top-level implementation approach on the practice of systems engineering unique to NASA. The material for updating this handbook was drawn from many different sources, including NASA procedural requirements, field center systems engineering handbooks and processes, as well as non-NASA systems engineering textbooks and guides.