The article deals with populism as analytical category applied to the trade union mouvement in Italy. Among the very different definitions of populism (some of which are not helpful for our topic) we adopt that of stressing populism as source of collective identity. Some very symbolic periods are discussed (the origins of Italian trade unionism, the fascism and the "autunno caldo") to show how some forms of populism can be viewed as a motive force of social processes instead of degeneration or "infantile disease". Populism can have a very strong gathering power and at the same time make social clevages stronger and deeper (insider and outsider, good and bad people, masters and workers). It and can more easily emerges when the need to spread potential mobilization base is particulary strong, i.e. when links and connections have to be underlined and divisions and differences have to be shadowed.