This chapter describes and explains trends in union structure and behaviour in the UK and makes some predictions about future trends. It differs from some other works on union structure in the following ways. First, it seeks to explain structural developments in terms of strategic issues facing trade unions in Britain. It takes a broadly Chandlerian view of structure in seeing it primarily as a dependent variable, an outcome of certain choices made by unions in response to exogenous and endogenous pressures. However, once chosen, such structures may exert a constraining effect on union behaviour. Second, it takes a slightly broader view of the term ‘structure’ than exists, for example in Clegg (1976): structure is viewed not simply as external morphology but also as embracing a system of internal exchanges and controls. While there is a clear distinction to be made between issues of structure and of democracy – topics which sustain very different literatures – this chapter will embrace all dimensions of structure as they relate to the ‘administrative rationality’ of unions, considering the ‘democratic rationality’ where necessary (Child et al. 1973).