Prophecies of doom for both working-class party and labor unions have gained popularity in the Western industrial democracies over the last two decades. The “old” Siamese twins, working-class party and labor unions, have a century-long history of their combined struggle to achieve political and industrial citizenship rights for the working class. Both forms of interest representation are seen as facing new challenges if not a crisis due to internal and external changes of both long-term and recent nature. However, despite these prophecies political parties and union movemehts have been differently affected and have responded in dissimilar ways across Western Europe. The Siamese twins, party and unions, as social institutions, their embeddedness in the social structure, and their linkages, were molded at an earlier time with long-term consequences. Hence, we cannot grasp today's political unionism, party-union relations and organized labor's capacity for change, if we do not understand the social and political conditions under which the organization of labor interests became institutionalized. An understanding of the origins and causes of union diversity helps us to view the variations in union responses to current challenges.