The automation of numerous command systems and/or process control systems, and the replacement of logical or analog systems involving little interaction by highly‐integrated systems, have led to a considerable expansion of the domain of functional safety, taking account of the features and peculiarities of computer systems. This chapter offers an examination of the software in the context in which it is used, which is a system, and recap on the links and the constraints which need to be taken into account in creating software. It lays down the vocabulary relating to the creation of a software‐based device. Software element is a set of computation/processing elements which are executed on a physical hardware architecture so that the system, as a whole, renders the services associated with a device. The chapter demonstrates the hierarchy which ranges from the system to the software, through the subsystems, devices and the electronic hardware.