Georg Simmel (1858-1918) is today not only a classic of sociology, who founded sociology as a science with his studies on the interactions between the individual and society. The later Simmel is also a philosopher of significance. This introduction provides an overview of the phases of Simmel's work, who began as a critic of Kant and idealistic metaphysics, made a name for himself as a sociologist and, above all, a cultural theorist, and turned in his later work to questions of aesthetics and a life-philosophical metaphysics. It becomes clear that Simmel consistently maintained certain questions throughout his work. Apparent contradictions in his oeuvre come together in a comprehensive view to form an open, fragmentary, and unfinished theory of modernity.