The human ability to adapt and innovate is both a strength and a weakness when it comes to operating complex systems. Safe operation requires a balance between the need to behave reliably in prescribed ways when things are operating in a way that the system designers foresaw, and the need to use judgement and experience to find a creative solution in a situation that no-one has previously seen or imagined. Rules and procedures play a key role in finding the right balance. This chapter explores the way in which rules both constrain and support operational managers when making safety-related decisions.