The mechanism of nucleophilic substitution by lower-order organocuprates depends in a profound way on the structure of the substrate, organocuprate, and reaction conditions. Early evidence suggested that a direct S2 displacement was occurring; however more recent results suggest that invertive oxidative addition of copper(I) into the carbon-leaving group bond takes place, generating a copper(III) intermediate which then undergoes reductive elimination to generate the coupled product. Both of these mechanisms predict inversion at the electrophilic carbon, which is observed in a number of cases. On the other hand, experiments with radical traps and the observation of racemization during substitution suggest a radical mechanism.