1. Introduction.- 2. Denjoy's Theorem and Exceptional Diffeomorphisms of the Circle.- 3. Full Diffeomorphism Groups Determine the Diffeomorphism Class of a Manifold.- 4. The C1 and C2 Theory of Diffeomorphism Groups.- 5. Chain Groups.- 6. The Slow Progress Lemma.- 7. Algebraic Obstructions for General Regularities.- 8. Applications.- A. Concave Moduli of Continuity.- B. Orderability and Hölder's Theorem.- C. The Thurston Stability Theorem.- Index.