This paper reviews the history of IBM contributions over the last quarter century to electronic structure theory. The two main sections of the paper deal with large-scale computations in quantum chemistry and the study of the electronic structure of solids. The paper briefly discusses the methods of ab initio quantum chemistry, a field in which IBMers have devoted major effort and made substantial innovative, scientific contributions. Partly as a result of these contributions, it is now possible to use these methods to address theoretically problems of practical importance in chemistry. Major contributions toward understanding the electronic structure of solids, particularly with regard to metallic bonding, surface electronic structure, the nature of magnetism, and semiconductor defects, are discussed as well as pioneering efforts in the analysis of photoemission and low-energy electron diffraction data.