During the early post-war period, Western trade union movements grew in membership and achieved an institutionalized role in industrial relations and politics. However, during the last decades, many trade unions have seen their membership decline as they came increasingly under pressures due to the social, economic and political changes. This article reviews the main structural, cyclical and institutional factors explaining union growth and decline. Concentrating on Western Europe, the empirical analysis compares cross-national union density data for 13 countries over the first period (1950-75) and for 16 countries over the second, ''crisis'' period (1975-95)
data(Ebbinghaus)