The state of systems of systems engineering knowledge sources Application of systems engineering to `systems of systems’ is a topic of increasing interest across the systems engineering community. This paper reviews the upcoming latest versions of three core SE knowledge sources, the Guide to the Systems Engineering Body of Knowledge (SEBoK), the INCOSE Systems Engineering Handbook and the SoS Annex to ISO/IEC 15288, Systems and softwareengineering – System Lifecycle Processes, and examines how they address various aspects of systems of systems engineering, as an indicator of the current state of knowledge on systems of systems and systems engineering.