This chapter explains the role of trade unions. It focuses on their public relations needs by examining what they do, the audiences they need to reach, and suggests what makes union communications effective. Modern trade unionism traces its history to the nineteenth century. Starting with skilled artisans, union expanded to less skilled workers, mainly men but women too with the match girls' dispute. A good measure of union effectiveness is density, the share of the workforce that is in a union. Charlwood estimates density in 1980 as more than half. Unions have multiple audiences, a range of interests and face preconceptions about unions. The size and power of trades unions has declined markedly since the middle 1980s but public attitudes towards them have become much more positive. Media coverage of union affairs has also declined but the stance of most newspapers in particular has remained negative.