This invention includes methods of forming layers comprising epitaxial silicon, and field effect transistors. In one implementation, a method of forming a layer comprising epitaxial silicon comprises epitaxially growing a silicon-comprising layer from an exposed monocrystalline material. The epitaxially grown silicon comprises at least one of carbon, germanium, and oxygen present at a total concentration of no greater than 1 atomic percent. In one implementation, the layer comprises a silicon germanium alloy comprising at least 1 atomic percent germanium, and further comprises at least one of carbon and oxygen at a total concentration of no greater than 1 atomic percent. Other aspects and implementations are contemplated.