In this paper, we look at the long term evolution of the composition of union membershipin the four largest European countries: France, West Germany, Italy, and the United King-dom. Using unexploited micro data coming from post electoral, labor, and household surveys,we first revisit commonly accepted unionization levels from the past 60 years. We find that,for France and Italy, union density was at time under- and over- estimated respectively. Sec-ond, we present long run evidence on the evolution of the composition of unions in terms ofthe socio-economic characteristics (occupation, length of education, public or private sector,gender) of their members. Two types of unionisation emerge from this analysis. In Franceand Italy, the composition of unions has been primarily determined by structural changesin the composition of the workforce with no notable changes in the selection of the differentgroups into unions when aggregate density varied. In the UK and West Germany, instead,selection into unions has changed dramatically: Blue collars and less educated worker wereover-represented in the ’60s, but this has declined over time. We argue that these two typesof unionization are related to the institutional characteristics of each country and show thatthe evolution of selection into union is linked to the public-sectorization of unions: as uniondensity fall, the share of public workers in unions increases.