This chapter links the distinguishing technological features of autonomous machines, identified in the previous chapter, to matters which are the subject of legal regulation: the decisions and actions of combatants, and the behaviour of weapon systems. It outlines the effects that weapon autonomy I likely to have on military decision-making processes and discusses the novel sources of risk and modes of failure associated with operating complex autonomous machines in combat. The problem of how to categorise autonomous weapons for legal purposes is addressed, as a response to suggestions by some commentators that weapon systems which are able to ‘step into the shoes’ of soldiers in some respects should perhaps be considered to be something more than just weapons when assessing their legal compliance. Finally, the conditions under which use of autonomous weapons may transgress the law are identified, as preparation for the detailed legal analysis to follow in the remainder of the book.