Ergonomic theory holds that accidents are preceded by periods of gradually increasing (but essentially unrecognised) risk, known originally in man-made disaster theory as the incubation period. This paper discusses the theorising of the dynamics of such accident incubation. It considers theoretical contributions - ranging from high-reliability to control theory to resilience engineering - for their ability to illuminate the driving forces behind a gradual shift in norms and erosion of safety margins, and for their ability to effectively track and represent such changes over time.