Today, there is no consensus, either in engineering sciences or in ergonomics, on the best way of introducing ergonomics–taking into consideration the human operator–into the design cycle of human–machine systems. Several authors agree in saying that the analysis and the design of complex, modern systems require an update of the methodological tools in relation to: the notions of “system” and “complexity”; and the effects of automation on the supervision of installations by the human operator. This chapter supports this notion. After presenting classic design approaches, the chapter looks at two methods that seek to integrate the different components of the system during design: the method by Long, Dowell and Timmer, which aims to define the performance criteria of the system that could orient the design choices, and the method developed by Rasmussen, then by Rasmussen, Pejtersen and Goodstein and by Vicente in a didactical form.