The borylation of alkanes and arenes has become some of the most practical C–H bond functionalization chemistry. Most striking is the high regioselectivity of these reactions. Rhodium and ruthenium complexes catalyze with exquisite selectivity the borylation of methyl C–H bonds over methylene or methine C–H bonds. Iridium complexes catalyze, with high steric control, the borylation of one aromatic C–H bond over another. In contrast, iridium-catalyzed borylation of heteroaromatic C–H bonds is more controlled by electronic effects. Detailed information on these selectivities and mechanistic information on the origins of this regioselectivity will be described in this critical review (95 references).