Over the last few years, an increasing number of scholars have adopted the so-called “ideational” approach to the study of populism. On the grounds of this approach, the populist phenomenon has been empirically investigated using a variety of different research strategies, including the classification by means of a minimal definition, content analysis, and expert surveys. In this article the main features of each different method are critically examined, and an overview of their pros, cons and pitfalls is provided, especially by comparing the research strategies aiming at classification, on the one hand, and measurement, on the other. I underline that the very rationale of the ideational approach relies in the need to operate a distinction between populist and non-populist actors through a minimal definition, which requires a classificatory effort analyzing the key ideological features of political actors in a qualitative fashion. At the same time, the other methods can only complement, but not replace, the qualitative analysis because they are not equipped to appropriately explore the core ideological features of the populist phenomenon.