In this essay Ronald Formisano emphasizes how problematic it is to give a unique definition of what populism is. Populism varies in fact according to national political cultures and has historically and politically been a different phenomenon in North America, in South America and in Europe. Before examining the different populist experiences in the United States, the author discusses the tendency in the European literature of the past twenty years to equate right-wing extremism and populism. Analysing United States populism, Formisano offers a broad overview of it, which goes from the experience of the People's Party of the early 1890s to Ross Perrot's challenge to democrats and republicans one century later. His conclusion is that in North America populism has been predominantly progressive, humane and directed toward social justice and has often been able to contain reactionary impulse and pressure. Making use of the broad literature on the topic and relying on the most recent scholarship, Formisano underlines how in the post World War II period populism began a migration from Left to Right.