This chapter describes the development of the European Union interest group population since the early 1990s. It assesses the more recent short-term ‘volatility’ (entry and exit) of the European Union population. The chapter seeks to provide several explanations for differences in the numbers of interest organizations per economic sector. When compared to lobbying in national capitals, the lobbying community of the European Union is studied extensively. Butt Philip provides the first comprehensive list of interest organizations in Brussels. Berkhout and Lowery note the major data challenges involved in the construction of a relevant census of active interest organizations that covers multiple European Union venues and that is consistent over time. They show that there has not been a recent ‘explosion’ of lobbyists in Brussels. Lobby experience implies that organizations build networks, learn through strategic trial-and-error and establish a positive reputation. This nurtures the capacity to affect policies and consequently makes residents likely to be more influential than tourists.