Political party think tanks (PPTTs) are important to the performance of parties' core functions. Despite their importance, they have largely escaped academic attention. To understand the role of PPTTs in contemporary politics, we develop a typology of their key functions (distinguishing between political party and think tank functions), and target audiences (distinguishing between internal and external target groups). Based on a comprehensive literature review and 22 in-depth interviews with leading representatives of PPTTs in the Netherlands and Flanders, we identify four types of PPTTs: Party Assistants, Party Supporters, Party Promoters and Party Intellectuals. The characteristics of the four types of PPTTs are illustrated through the analysis of four paradigmatic cases: the Study Centre Open Vld as an example of a Party Assistant, the Scientific Bureau GroenLinks as an example of a Party Supporter, the Renaissance Institute as an example of a Party Promoter, and the TeldersFoundation as an example of a Party Intellectual.