The history of popular education is a long-standing field of research. Its beginnings trace back at least to the mid-twentieth century and it maintains its attractiveness for many researchers, both historians and educators. But, using the same label, quite different lines of research have been developed. They correspond to different ideas about what has to be considered as popular education. The paper starts by revising some of the outstanding works and authors in this field, analysing the main terms used and their underlying concepts. Popular education is then confronted with other close concepts, such as people education, popular or working-class self-education, adult education, women's education or social education. The author argues that the approach and perspective adopted for identifying a popular education subject of research are more determinant factors than an "a priori" thematic demarcation. From this point of view, popular education is centred on the study of education activities and school provision addressed to the lower classes and individuals taking part in them. This social origin and position of the public and not of the providers is what really defines this field of study. (Contains 43 footnotes.)