Astronomer and Marxist Anton Pannekoek was a remarkable f igure. As an astronomer, he pioneered quantitative astrophysics and founded the renowned Astronomical Institute in Amsterdam that now carries his name. Before World War I, however, he was employed as a Marxist theorist by the Social Democratic Party of Germany, making him one of the leading intel-lectuals of international socialism. Because of his signif icant contributions to such diverse subjects as astronomy and socialism, Pannekoek’s life and work uniquely capture the fascinating connections between conceptions of nature, society, and their representations in the early decades of the twentieth century. This book aims to study these connections through the prism of Pannekoek’s biography. In doing so, it sets out to explain Pan-nekoek’s particular epistemic, aesthetic, and political choices, while placing them in the broader context of the early twentieth century.