This chapter is about emergent safety hazards in engineering systems. These
hazards are those that emerge from a system without arising from any part of the
system alone, but because of interactions between parts. We distinguish two
approaches to analysing engineering systems: one is to view them as sociotechnical, and the other is to consider them as cyber-physical systems. We
illustrate a great deal of emergent hazardous behaviours and phenomena due to
unknown accident physics, malign actions, chemistry, and biology and due to
deficiencies in managements and organisations. The method that follows the
socio-technical view consists in the representation of a system by sequential
functionally unrelated processes that can in reality influence the performance of each other via sneak paths. The method that follows the cyber-physical systems
view focuses on the analysis of control loops (feedback, feedforward, positive,
and negative) and, especially, interrelated loops. The chapter explores also the
realm of security threats due to malign actions that can trigger safety-threatening events. And finally it gives general guidance for avoiding and eliminating safety hazards when designing engineering systems.