The Atiyah-Singer index theorem, a landmark achievement of the early 1960s, brings together ideas in analysis, geometry, and topology. We recount some antecedents and motivations, various forms of the theorem, and some of its implications, which extend to the present. Contents 1. Introduction 1 2. Antecedents and motivations from algebraic geometry and topology 2 3. Antecedents in analysis 9 4. The index theorem and proofs 12 5. Variations on the theme 17 6. Heat equation proof 24 7. Geometric invariants of Dirac operators 29 8. Anomalies and index theory 38 About the author 45 References 45