Metal-catalyzed addition of P-H bonds to alkenes, alkynes, and other unsaturated substrates in hydrophosphination and related reactions is an atom-economical approach to valuable organophosphorus compounds. Understanding the mechanisms of these processes may enable synthetic improvements and development of new reactions. The first step in several catalytic cycles is P-H oxidative addition to yield intermediate metal hydride complexes bearing M-P bonds. P-C bond formation may occur via substrate insertion into the M-H bond, followed by P-C reductive elimination, or by insertion into the M-P bond and C-H reductive elimination. In an alternative outer-sphere process, nucleophilic attack of a metal-phosphido (M-PR2) group on an unsaturated substrate and proton transfer involving the metal hydride yields the product. This Perspective reviews the mechanistic possibilities, with a focus on the P-H activation step, and recent progress in developing novel catalytic transformations involving P-C bond formation.