This chapter introduces the problem of official action beyond statute as a central concern to public law theory. It reviews the most important classical accounts of the Royal Prerogative and the modern notion of a ‘third source’ of executive power besides statute and prerogative. It also introduces a major them in the book, namely the concept of ‘official power’ as a set of ‘legal powers’ and uses this critically to assess common misconceptions about the nature of official action beyond statute – particularly conceptions of the third source as a set of ‘residual liberties’ inuring in the Crown in common with private individuals.