Socialists have been deeply involved in the union movement from its very beginnings. There is an equally long history of debating exactly how socialists should be involved in the union movement. Should socialists try to win elected office in unions? Should they join the paid staff of unions? Does the leadership of unions represent different social interests than rank-and-file workers? While some of the answers to these questions depends on a number of different variables, including specific histories of particular countries and the balance of power between labor and capital, there are longer standing trends that we need to understand in order to be successful at organizing fights against the boss and winning unionized workers to the socialist movement. We will also discuss how different socialist tendencies have approached these questions and to what ends.