It has been about a hundred years since the atomic nature of matter began to be generally accepted. Even Einstein’s PhD thesis (1905) on the size of a sugar molecule had doubters. However, by the late 1920s, the atomic theory was well established, and quantum theory had explained many properties of atoms in gases. In particular the interpretation of the sharp lines in atomic optical spectra could be explained in terms of transitions between electronic energy levels. The application to interacting atoms in solids appeared straightforward in principle, and although the quantum theory did answer many fundamental questions about condensed matter, the theoretical applications were mostly appropriate for idealized models of solids. Because the optical spectra of solids had broad peaks, explaining their origin in terms of electronic transitions was much more difficult than for the case of atoms. In fact, it wasn’t until the 1960s when accurate electronic band structure calculations could be done for real materials. I plan to describe this evolution in the application of quantum theory to condensed matter. I’ll focus on basic and applied research involving semiconductors, superconductors, and nano-structured materials. These are areas where the use of quantum theory has been central in both explaining and predicting properties and has even led to the discovery of new materials.