The synthetic potential and the mechanistic aspects of inorganic insertion reactions of carbon monoxide, especially into metal-carbon σ-bonds, are considered. Reactions of this type are encountered among most 3d, 4d, and 5d elements. In one case it has been demonstrated that the CO insertion proceeds by alkyl migration; this is likely to be a general feature of all such reactions. If an alkyl migration takes place, then insertion of CO into the MC bond is governed kinetically by the cleavage of that bond. CO abstraction from RCOM bonds most commonly proceeds by rate-determining vacation of a coordination position. Both CO insertion and abstraction are usually highly stereospecific.