In the wake of Bernie Sanders remarkable performance in the 2016 U.S. presidential primaries, democratic socialism captivated the hearts and minds of many voters on both sides of the North Atlantic. Less than a decade later, however, the enthusiasm is gone, and democratic socialism faces an intellectual crisis. This book asks which insights the history of democratic socialism may render to resolve this crisis. For this purpose, it investigates the theories and practices of four democratic-socialist movements: the cooperative one of the 1830s-50s, the collectivist one of the 1900s-20s, the neo-collectivist one of the 1960s-80s, and the participatory one of the 1990s-2010s.