The article analyses three recent books - by Yves Mény and Yves Surel, Guy Hermet and Paul Taggart - on populism. It is argued that all these books, although from different perspectives, interpret populism as a form of weak ideology with a strong core. This core - which is adaptable to different contexts - consists of a conception of democracy as the direct derivation of the will of the "people". The latter are perceived as a monolithic and homogeneous unit which express their view in a sort of "resemblance democracy", often personified by a charismatic leader. By analysing the different forms which populism has taken in various areas and at different times, it is possible to identify the main causes of the phenomenon. The first is the emergence of a deep socio-economic crisis, which produces a feeling of high uncertainty and a loss of identity among the lower classes. The second is the perception of an intolerable discrepancy between the desired and the actual ways in which democracy works. In some ways populism is intrinsic in democracy and can even act as an antibody that helps it to regenerate; on the other hand, it can also behave as a virus when its aversion for pluralism prevails. The article also examines the different expressions of populism throughout history and in different geographical contexts, focusing on features of Western European new populism and especially on the Italian case.