The impressive evolution of diboron reagents in recent years has paralleled the development of diboration reactions. Since the discovery of the uncatalyzed diboration reaction, new methods to activate diboron reagents with transition metal complexes and Lewis bases provide efficient ways to catalyze the diboration of unsaturated substrates with total control of regio-, chemo-, and enantioselectivity. This rapid progress is supported by theoretical calculations that shed light on the mechanisms involved and stimulate the development of new boron–boron activation modes towards geminal and vicinal diboration of carbon–carbon multiple bonds. Overall, the diboration reaction represents an improvement over alternative strategies to generate two carbon–boron bonds, such as double hydroboration or double dehydrogenative borylation. Selective diboration protocols are highlighted in the efficient syntheses of target compounds such as exo-brevicomin, (+)-discodermolide, (S)-fenpropimorph, and (Z)-tamoxifen.