This article assesses the size and diversity of Belgium’s interest group population by triangulating four data sources. Combining various sources allows us to describe which societal interests get mobilised, which interest organisations become politically active and who gains access to the policy process and obtains news media attention. Unique about the project is the systematic data collection, enabling us to compare interest representation at the national, Flemish and Francophone-Walloon government levels. We find that: (1) the national government level remains an important venue for interest groups, despite the continuous transfer of competences to the subnational and European levels, (2) neo-corporatist mobilisation patterns are a persistent feature of interest representation, despite substantial interest group diversity and (3) interest mobilisation substantially varies across government levels and political-administrative arenas.