The purpose of this chapter is to rehearse the theoretical basis for the concept and practice of radical political unionism and to contrast this concept with broader definitions of social movement unionism (SMU). In doing so the chapter draws on social movement theory and relates such theory to the question of trade union organising. The chapter draws on articles written by the authors, which are referenced in the text, and follows the publication in 2009 of the book “The Crisis of Social Democratic Trade Unionism in Western Europe: The Search for Alternatives”. The authors focus primarily on the crisis of trade unionism in Europe across each of its main dimensions that is manifested as a crisis of membership and mobilisation, a crisis of identity and legitimacy and a crisis of political representation. It then attempts to utilise aspects of social movement theory to develop an alternative framework of radical political unionism as a possible way out of this crisis.