We report magnesium-catalyzed cross-dehydrocoupling of Si-H and N-H bonds to give Si-N bonds and H-2. A number of silazanes are accessible using this method, as well as silylamines from NH3 and silylhydrazines from N2H4. Kinetic studies of the overall catalytic cycle and a stoichiometric Si-N bond-forming reaction suggest nucleophilic attack by a magnesium amide as the turnover-limiting step.