This chapter explores the conceptual foundations and historical trajectory of the Modern Synthesis (MS), the framework that united Darwinian natural selection with Mendelian genetics to form the cornerstone of modern evolutionary theory. I critically examine the MS’s theoretical commitments, highlighting its ambiguities and inner tensions, particularly the balance between mechanistic and teleological explanations in biology. Key themes include the role of adaptationism, the debate over whether natural selection is creative or eliminative, and the unresolved question of variation’s origin. The chapter also interrogates the MS’s narrative coherence, challenging the notion of a seamless Darwin-Mendel partnership and exposing the compromises that shaped the synthesis. By engaging with critiques from mutationists, neutralists, and developmental biologists, the chapter sets the stage for later discussions on extending or replacing the MS, emphasising the need for a more inclusive evolutionary theory that accounts for organismal agency, plasticity, and process-oriented ontologies.