Recently, scholars in several European countries have initiated system-wide, population ecological studies (e.g. Halpin & Jordan 2011b, Messer, Berkhout & Lowery 2010) or have started data collection on such populations of interest organizations (e.g. Fisker 2013, Klüver 2012, Naurin & Borang 2012). In this review I assess the longer running research traditions of large-n studies, recently published ecological studies and some projects that are still on-going. I compare these studies on their implicit or explicit assumptions about the ‘competitive environment’ of the organizations studied and how these assumptions are included in the research design. The main distinction in the literature is between, on the one hand, researchers who assume that interest organizations of a certain type such as social movement organizations or business interest associations compete for organizational resources, and, on the other hand, researchers who assume that interest organizations active on a certain topic, policy field or economic sector compete for organizational resources. The review concludes with an assessment of specific empirical and theoretical challenges in various strands in the literature, opportunities for fruitful comparisons of research findings of different projects and venues for further research.