Analysis of the structures in the Protein Databank, released in June 1996, shows that the number of different protein folds, i.e. the number of different arrangements of major secondary structures and/or chain topologies, is 327. Of these folds, approximately 25% belong to the all-alpha class, 20% belong to the all-beta class, 30% belong to the alpha/beta class, and 25% belong to the alpha + beta class. We describe the types of folds now known for the all-beta and all-alpha classes, emphasizing those that have been discovered recently. Detailed theories for the physical determinants of the structures of most of these folds now exist, and these are reviewed.