Populism has become pervasive in political language and in the diagnosis of the malaise of contemporary politics. At the same time, more narrow definitions of populism have become shared in scholarship on the subject, nourishing more analytical approaches that put populism in historical and cross-regional perspective. The purpose of this introduction is to use the evidence assembled in this special issue to ask some fundamental questions concerning the study of populist mobilization. Most importantly, what do we gain and what do we lose from sight by focusing on the commonalities between parties based on their populist appeals, when populist parties differ dramatically in terms of the substantive ideologies they adhere to? Are there distinctive features in terms of voter attitudes that underlie populist mobilization? And if failures of political representation and populism are intimately related, can we expect populists to render party systems more responsive to voters' substantive policy preferences?As populism seems to become ever more widespread in different regions of the world, there is a growing consensus on a minimal definition of the phenomenon that centers on its ideological traits. In Mudde and Kaltwasser's (2013) terms, populism is a "thin ideology" that builds on the juxtaposition of the elite and the people. Employing this discourse enables charismatic leaders to mobilize citizens who no longer feel represented by established political actors. The aim of this special issue is to gauge the contribution of the concept of populism to our understanding of political mobilization in the recent period and to compare these instances of populist mobilization to earlier ones. While more and more phenomena are looked at through the lens of populism, it is important to know how important this thin ideology is compared to the host ideologies that these parties or movements almost invariably also exhibit, and thus how similar they really are. For example, how much do we gain in looking at the populist radical right in terms of the concept of populism, rather than theories from the established literature on the radical or extreme right?The growing consensus on what populism constitutes has the advantage of allowing for comparisons across time and space. While populism can be associated with various more substantive "host ideologies" such as socialism or cultural traditionalism and nativism, a common definition allows us to compare its left-wing, right-wing, and more "pure" manifestations that fail to exhibit clear-cut affinities to host ideologies. How far, then, do the commonalities between left-wing and right-wing populist mobilization go? The aim of this exercise is not to proclaim either the populism lens or more classical approaches as superior to the other, but rather to contribute to a fruitful cross-fertilization or integration of approaches. Certainly, in looking through the lens of populism, we should not forget Swiss Political Science Review 23(4): 301-312