Though parties serving as “personalist vehicles” have been known to exist for some time in less well-established democracies, they are certainly less commonplace in well-institutionalized democracies where parties are normally expected to make decisions by routinized, democratic procedures and maintain substantial value in their own right. And yet, even highly institutionalized parties in such settings may fall prey to personalistic tendencies.1 Such has, for instance, been the case recently for one of the most established, institutionalized parties in one of the most established, institutionalized democracies in the world: Donald Trump's Republican Party in the United States. And while that case may be the most notorious of late, it is hardly the only instance of this phenomenon! Indeed, across a range of democracies over a span of decades, there have been numerous other cases of personalization of well-established parties, though not always personalized in exactly the same ways or to the same degree. It is our explicit purpose in this special issue to gain a better understanding of numerous relevant cases and the process of party personalization in general, through a collection of case studies rigorously employing a common conceptual framework and guided by similar research questions. To what degree, and in what ways, has each case experienced personalization? What factors and circumstances made this possible? (And to the process culminated in less than complete personalization, what hindered/stopped the process?) What have been—or are likely to be—the consequences for the party and the polity?