Research on party membership development commonly reports figures aggregated to the country level and/or using only a few time-points. While these choices may be appropriate for certain research questions, they nevertheless hide major differences between parties and conceal short-term fluctuations. Additionally, they are inappropriate for studying individual party trajectories. This is necessary, however, to better describe and ultimately explain the phenomenon of membership decline. The article analyses in total 1653 observations across 47 parties in six western European countries between 1960 and 2010 to test hypotheses pertaining to individual party membership development. Using multilevel modelling and time-series analyses, the results show what aggregated data with few time-points cannot: membership decline is by far not a universal phenomenon. Additionally, membership decline appears to be part of a party’s life-cycle. The more consolidated parties are, the fewer members they have. Few differences between party families are observable.