This chapter sheds light on the role of political parties as social investment protagonists, consenters, or antagonists in the reform of labor market and family policies in Greece, Italy, Portugal, and Spain. Drawing on original, hand-coded data of three decades of labor market and family policy reforms in Southern Europe, the findings show divergent social investment trajectories. While Spain and Portugal have started to develop contours of a social investment agenda, little progress has been made in Italy and Greece. Programmatic political competition and government partisanship play a role in accounting for these divergent trajectories. Center-left parties have acted as the primary social investment protagonists in Spain, Portugal, and Italy. However, the Italian center-left remains fragmented and has rarely been in government. In stark contrast, both center-right and center-left parties in Greece have acted as social investment antagonists. Political and economic turmoil in the wake of the Eurozone crisis paints a bleak picture for the further development of social investment in Southern Europe. Once fiscal constraints can eventually be overcome, a core question remains to what extent an inclusive social investment coalition can be formed in an ever more fragmented political landscape.