This chapter focuses on presentations given at the 20th International Symposium on Military Operational Research and NEC – The Human Dimension combined with reflections on the presentations and discusses from Human Factors of Decision Making in Complex Systems. It explores the need for Operational Research (OR) studies to embrace the broad range of variability arising from human participation in the operations and systems studied, and the consequent need for the human science communities to integrate and present their knowledge in forms which are amenable to exploitation by OR practitioners. The wide spectrum of sources of variability in systems outlined above demands a response from OR, if the discipline is to retain its credibility. OR's practical response to the challenge of human variability should include a broadening of scope, at all stages of the analysis process, to cover the full spectrum of significant factors.