The reaction mechanism of decomposition of diazocarbonyl compounds with copper begins with the formation of a copper carbene complex. Evidence for the formation of copper carbenes is provided by comparison to the behavior of photolytically generated free carbenes and the observation of appreciable enantioselectivity in cyclopropanations with chiral copper complexes. Upon formation of the copper carbene, either insertion or addition takes place to afford carbocycles or cyclopropanes, respectively. Both addition and insertion proceed with retention of configuration. Thus, diastereoselectivity may often be dictated by the configuration of the starting material.