This chapter explores the human contribution to the safety or otherwise of complex well defended systems. It examines some of the behavioural options available to people working in complex technologies. The chapter discusses both organizations and human behaviour is complex, a bewildering number of distinctions. Administrative controls range from the mainly prescriptive to the largely discretionary. In the early stages of a technology or manufacturing process, work will probably be governed largely by trial and error, or by feedback-driven controls. Military cultures differ in the autonomy granted to junior commanders and non-commissioned officers. The main distinctions between three levels of human performance: the skill-based (SB), rule-based (RB) and knowledge-based (KB) levels. Thus, Human error can be defined as the failure of planned actions to achieve their desired ends without the intervention of some unforeseeable event.