Until recently the use of organometallics as discrete reagents and intermediates in organic synthesis had been largely restricted to a relatively few classes or compounds, such as those containing lithium and magnesium. Mainly within the last two decades, organocopper compounds and organoboron compounds have emerged as versatile and useful reagents and intermediates in organic synthesis. Except for carboranes, organoboron compounds generally exist either as tricoordinate or as tetracoordinate species. The trisubstituted derivatives of boron are called boranes, which may exist either as essentially trigonal planar monomeric species or as aggregates in which the boron atoms occupy the central position of an essentially tetrahedral configuration. In fact, essentially all monoorganoboranes (RBH2) and diorganoboranes (R2BH) as well as the parent borane (BH3) exist as the dimers, whereas triorganoboranes (R3B) are usually monomeric. For the sake of simplicity, however, all of these compounds are treated as monomers in this chapter, except when such a simplification is unacceptable. This chapter describes some of the key features common to various organoboron reactions used for the formation of carboncarbon and carbonheteroatom bonds and summarizes the available information on these reactions according to the product types. Hydration of olefins and acetylenes via hydroboration–oxidation was reviewed earlier, and this chapter does not update this reaction. Also excluded is a detailed discussion of the synthesis of olefins and acetylenes via organoboron compounds.