PURPOSE. The purpose of this paper is to outline Situational Action Theory (SAT) and its Developmental Ecological Action Model (DEA model) as applied to the explanation of criminal careers. The DEA model of SAT was first presented in Wikström (2005) and subsequently refined in Wikström and Treiber (2019) and is further elaborated in this paper. METHOD. The central argument of the DEA model of SAT is that if we want to explain stability and change in people’s crime involvement we first have to understand what factors and processes move people to commit acts of crime. Only then can we adequately assess what factors and processes are involved in the explanation of criminal careers and people’s differential pathways (trajectories) in crime. RESULTS. The DEA model of SAT address some of the main limitations of current dominant explanatory approaches in Developmental and Life-Course (DLC) Criminology (Wikström & Treiber, 2009) and champions a general, dynamic and mechanism-based account of the causes of crime (Wikström, 2017) and the drivers of criminal careers (Wikström, Treiber & Roman, 2019). It integrates key insights from two great but poorly integrated traditions in the study of crime and its causes: the individual/developmental and ecological/environmental traditions.