Can the recent decline in union density in Europe be attributed to specific economic, social or institutional causes? Can unions influence these causes and reverse decline? Using two data sources — a representative survey of Dutch employees and a data set for European countries between 1950 and 1997 — the author examines the determinants of union decline. The theoretical model is based on a social custom approach to unionization, integrating rational choice and social network theory. The empirical results show improbable small margins of union resurgence, that is, if institutional support for union representation can be maintained or regained within and beyond workplaces.