The history of the Socialist Party (SP) starts with the establishment of the Communist Union Movement Netherlands/Marxist-Leninist (KENml). According to a former prominent SP member Koos van Zomereren, party membership entailed sacrificing one's freedom, old friendships, leisure time and money. Inspired by the democratic centralism of Lenin, internal discussion was kept to a minimum within the party. This chapter explores origins and ideology of the SP. By 1994 the SP obtained its first parliamentary seats with the slogan 'Vote against, vote SP!' Under the lead of Jan Marijnissen, the party became an established actor of the Dutch party system even though it has never gained governing power so far. The chapter also explores socio-demographic and attitudinal profile of the SP voters in 1998, 2002, 2006 and 2010, and the electoral fate of the SP. Most of the facilitating factors for populist parties also contributed to the SP's success.