Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Donaldson theory is the study of smooth 4-manifolds using gauge theory. It was started by Simon Donaldson (1983) who proved Donaldson''s theorem restricting the possible quadratic forms on the second cohomology group of a compact simply connected 4-manifold. Most of the results of Donaldson theory depend on the manifold having a differential structure, and are false for topological 4-manifolds. Many of the theorems in Donaldson theory can now be proved more easily using Seiberg Witten theory.