The main goal of this article is to analyse the role of social, political and ideological factors in explaining party system change in Portugal during the democratic period (April 1974June 2005). We begin with the evolving trends in the party system format, and their main effects on the patterns of parties' interactions as well as on the functioning of the political system. This first approach will enable us to divide the democratic period into two distinct phases and an interlude. Following this, we will analyse the dimensions and strength of policy competition between the parties before, finally, analysing the impact of social, political and ideological factors on party system change in Portugal.