In this analytical chapter, political scientist Karl Magnus Johansson evaluates the European People’s Party’s (EPP’s) capacity to shape European integration through treaty reform and political coordination. Drawing on three decades of research and case studies, he argues that Europarties such as the EPP can exert real influence—but only under demanding conditions: numerical strength, internal cohesion and the ability to mobilise networks of party leaders. Johansson traces how the EPP played a decisive role in the Single European Act and the Maastricht Treaty but struggled during the Amsterdam Treaty negotiations due to internal divisions and weakened Council presence. He concludes that while the EPP has shaped the EU’s constitutional architecture, its effectiveness depends on unity, strategic mobilisation and clarity of purpose—lessons that remain vital as the party navigates today’s fragmented landscape.