While there are some signs of revitalization, social democracy has witnessed a deep electoral crisis over the last decades. The causes for the decline of social democratic parties are highly contested among researchers. This article provides a systematic review of the literature which spans several fields such as party politics, political sociology and political economy. Four kinds of explanations (sociological, materialist, ideational and institutional) are distinguished and scrutinized on the basis of empirical studies published since 2010. The findings indicate that there is not one explanation that stands out but that the electoral crisis of social democracy is a complex phenomenon with multiple causes, such as socio-structural changes, fiscal austerity and neoliberal depolarization. In addition, the findings suggest that a liberal turn on sociocultural issues does not necessarily lead to vote losses. Further research should explore more deeply how short-term and long-term factors have worked together in the process of social democratic decline.