An empirical analysis of various waves of the ALLBUS social survey shows that union density fell substantially in West Germany from 1980 to 2000. Such a negative trend can be observed for men and women and for different groups of the workforce. Repeated cross-sectional analyses suggest that a number of personal, occupational and attitudinal variables such as sex, occupational status, firm size and political orientation play a role in the unionization process, although the influence of some variables is not robust over time. While the results are consistent with cost-benefit considerations on the sides of employees and unions, our estimations do not support individualization theory and they cast some doubt on a crucial prediction from social custom theory. © Oxford University Press and the Society for the Advancement of Socio-Economics 2005. All rights reserved.