The Conference on Groups and Geometries held in Siena in 1996, addressed a broad range of topics in group theory and geometry, with emphasis on recent results and open problems. Special attention was drawn to the interplay between group-theoretic methods and geometric and combinatorial ones. Expanded versions of many of the talks appear in these proceedings. Algebraists and geometers will encounter in this conference record a collection of ideas stemming from work in such areas as: the classification of finite simple groups; the structure and properties of groups of Lie type over finite and algebraically closed fields of finite characteristics; buildings, and the geometry of projective and polar spaces; and geometries of sporadic simple groups.