Transition-metal oxides and sulfides can be insulators, semiconductors, or metals, or undergo metal-nonmetal transitions. The experimental situation in these materials is briefly reviewed. Pure Bloch-Wilson theory predicts that many of the insulators should be metallic, so this class cannot be explained in terms of pure band theory. However, it now appears likely that band theory can be adapted to describe most of the materials that exhibit metal-nonmetal transitions.