How do electoral systems and intraparty candidate selection procedures affect party unity? In this article, I distinguish theoretically and empirically between electoral systems and candidate selections and argue that the influence of selection processes on parties' behavior is conditional on electoral system (and vice versa). Measuring parties' unity using Rice and weighted Rice scores, and applying hierarchical models to a new data set of 249 parties in 24 countries, I find support for the claim that the influence of selection processes on behavior is greater under electoral systems that encourage personal vote-seeking incentives than under electoral systems that encourage party centeredness.Maintaining a unified party voting record is an important objective for both party leaders and rank-and-file members. Unified parties aid policy implementation and improve stability in both parliamentary and presidential systems. Without party unity, in presidential systems, the president's base of legislative support becomes less stable and predictable, whereas in parliamentary systems government survival might be threatened. Additionally, a unified party record helps enhance the party's electoral performance, as it creates a clear brand name with which voters identify. Lastly, within the legislative arena, a unified party record reduces transaction costs in log rolling, increases policy-making capabilities, decreases legislative unpredictability, and amplifies legislative efficiency. Indeed, disciplined-unified parties are regarded as one of the pillars of the responsible party government doctrine (Ranney 1954).What explains the degree to which parties behave as a unified actor? Scholars have long argued that electoral systems and intraparty candidate selection processes (i.e., the manner by which a party decides who can represent it on its ballot) shape the balance legislators strike between emphasizing their unique personal reputation and the collective LEGISLATIVE STUDIES QUARTERLY, 00, 00, month 2016