Graham Mathieson was a Fellow of the UK MOD's Defence Science and Technology Laboratory where he was a member of the Human Systems Team. He was an innovative thinker who made significant contributions to the understanding and analysis of military command and control, with particular emphasis in recent years on the proper representation of humans in models. His work had an impact not only in the UK, but worldwide via NATO working groups, the DOD's CCRP, and collaborative international projects. He was a major contributor to the NATO Code of Best Practice for C2 Analysis. Graham was passionate about the rigorous use of science to inform decision making, and was well known for challenging his colleagues' ideas with constructive arguments that were based on his own clear and innovative thinking. This volume brings together selected works from Graham's published writings on complex human systems in military operations research, which should be of interest to human systems researchers in both the military and civilian domains. Part I ("Scoping the Problem Space") brings together three chapters that represent Graham's later works on complexity thinking and how it might be applied to human organizations. The extent to which he embraced the new complexity paradigm is clear, and how he was beginning to weave it into the needs of OA customers. Part II ("Confronting the Problem Space") contains papers that are much less general and abstract, and address specific issues in the complex world of human systems modeling.