Treating interest groups mainly as independent units of observation overlooks highly frequent coalition activities between actors and risks affecting the results of studies of lobbying and political influence. Yet, conceptualising and measuring lobbying coalitions is inherently difficult. In order to facilitate important future research, this article provides a roadmap of the main conceptual and methodological choices involved in studying lobbying coalitions. It distinguishes three main approaches to identify coalescing actors: a preference similarity approach, a behavioural approach, and an organisational approach. The article presents concrete operationalisations of coalitions from these vantage points and provides empirical evidence that various forms of cooperation activities on specific issues, as well as general cooperation structures, are highly frequent in lobbying in European countries. The article is relevant for scholars of interest groups and political advocacy more broadly by informing the design of new research on lobbying strategies, access, or influence.