How are the contours of Western European politics shifting? To what extent do these shifts reflect changes in the underlying social and economic structure of European polities? In this article, we reflect on insights from the classic literature on how cleavages structure party systems and consider how the emergence and persistence of new parties and new ideological conflicts are leading to both shifts of dividing lines of party competition and the fragmentation of party systems. While increasing attention has been given to the so-called second dimension of European electoral politics, we highlight the relatively limited focus on structural changes that are helping to drive this transformation. We identify some socio-demographic developments that are potentially generating new cleavages in Western European democracies: the expansion of higher education; mass migration and the growing ethnic diversity of electorates; the aging of societies and sharpening of generational divides; and increased geographical segregation of populations between prospering, globalized major cities and declining hinterlands.