We are interested in learning about the microscopic mechanisms that lead to the occurrence of collective effects in solids such as superconductivity and ferromagnetism. We believe that the essential physics of these phenomena is likely to be contained in simple models, even if the materials where these phenomena occur are very complicated. Once a plausible model is identified, we study it using various analytic and/or numerical techniques to learn about its properties and compare with experimental observations. For example, we have recently found that electron-hole symmetry breaking, i.e. electron-hole asymmetry, is likely to play a fundamental role in superconductivity, and are studying the consequences of this for optical and tunneling experiments.