This chapter describes the synthesis and applications of four-coordinate pentavalent organophosphorus compounds with a PSe group, including: (i) the addition reaction of secondary phosphine selenides; (ii) Woollins’ reagent as a selenating agent and its derivatizations to the compounds containing phosphorus and selenium atoms; (iii) chiral phosphoroamidites as Lewis base catalysts; and (iv) PSe compounds with a binaphthyl group as a chiral transfer reagent.