In my group, we try to understand real problems in corrosion, lubrication, heterogeneous catalysis, and microelectronics (thin films) by creating simple model surfaces and studying their chemistry on an atomic scale. We then extrapolate this information back to the real system, where such knowledge is often unobtainable, either because of a lack of adequate experimental techniques or because of misleading extraneous factors. In order to create a simple model of a surface chemical process, we use single crystal surfaces with known morphology. Ultrahigh vacuum provides the well-controlled chemical environment necessary to study a reactive surface over a period of several hours. We can deliberately expose the sample to a chosen gas in order to change its surface composition and then characterize it.