Catalysis by soluble complexes of transition metals is a rapidly gaining mode of catalysis in organic synthesis. These metals form bonds with one or more carbons in an organic reactant resulting in complexes that are known as organometallic complexes. Catalysis by these complexes is often referred to as homogeneous catalysis. Among the important applications of homogeneous catalysis in organic synthesis are isomerization of olefins; hydrogenation of olefins (carried out using Wilkinson type catalysts); oligomerization; hydroformylation of olefins to aldehydes with CO and H2 (the oxo process); carbonylation of unsaturated hydrocarbons and alcohols with CO (and coreactants such as water); oxidation of olefins to aldehydes, ketones, and alkenyl esters (Wacker process); and metathesis of olefins (a novel kind of disproportionation). Enantioselective catalysis that rivals enzymes in selectivity is a major development in homogeneous catalysis. As a result, many earlier processes in the pharmaceutical and perfumery industries are being replaced by more elegant syntheses using soluble catalysts in which “handedness” is introduced in the critical step of the process, thus avoiding the costly separation of racemic mixtures. In view of its importance in organic synthesis, enantioselective (or asymmetric) catalysis was briefly introduced in Chapter 6 and is again considered as a powerful synthetic tool in Chapter 9. This chapter is concerned with the use in general of homogeneous catalysis in organic synthesis (including asymmetric synthesis). Among the several books and reviews written on the subject, the following may be mentioned: Halpern (1975, 1982), Bau et al. (1978), Parshall (1980), Masters (1981), Collman and Hegedus (1980), Eby and Singleton (1983), Chaudhari (1984), Davidson (1984), Kegley and Pinhas (1986), Collman et al. (1987), Parshall and Nugent (1988), Noyori and Kitamura (1989), Parshall and Ittel (1992), Gates (1992), Chan (1993), Akutagawa (1995). Gas (or liquid)-phase reactions on solid catalysts are among the most common industrial reactions. However, homogeneous catalysis is rapidly catching up. Excluding applications in petroleum refining, the dollar value of organic chemicals produced worldwide by homogeneous catalysis (more than $35 billion) is quite impressive compared to that by heterogeneous catalysis (more than $45 billion). Attempts are now under way to find an integrated approach to homogeneous and heterogeneous catalyses (Moulijn et al., 1993).