This chapter outlines a model of Senior Investigating Officer (SIO) decision-making in murder inquiries. Specifically, we focus on: (i) 'Decision environment' (including intra-organisational features such as accountability, hierarchy and external pressures such as publicity, type, stage and area of investigation); (ii) the individual 'Decisionmaker' (including the ultimate accountability of the SIO) and whether he or she has a participatory or autocratic role); and (iii) 'Decision bases' (the material foundations of particular decisions - from hunches to evidence). Our model is based on the attendant psychological and sociological literature, as well as examples from the debriefs we have examined and a variety of archival resources (notably public reviews). We consider the psychological impact of time pressure, uncertainty, responsibility, reversibility and control, and evaluate how these may be more or less relevant at various stages of an inquiry. In drawing upon these various levels of complexity that influence decisions, we argue that both naturalistic and traditional decision-making approaches are informative within the context of critical incidents and suggest that a synthesis of these historically 'at odds' paradigms is likely to prove most fruitful in furthering our understanding of the role of the SIO in murder investigations.