This chapter focuses on the hydrogenation reactions catalyzed by transition metal complexes, with the aim of developing catalysts for selective hydrogenation under mild conditions. Olefin hydrogenations—for example—are important industrial processes, and selectivity is critical to the success of such processes. Greater product selectivity has an important impact on energy and resource utilization in terms of reduced process energy requirements for product separation and purification, and in terms of low-value byproducts. The advances in asymmetric hydrogenation—a stereospecific selectivity—have been notable, and an understanding of the detailed pathways is just beginning to emerge, although much remains to be done, and matching of substrates with the most suitable chiral catalyst still remains an empirical art. The chapter demonstrates the degree of understanding that can be attained for a homogeneous hydrogenation catalyst at the molecular level. Enantiomeric products are used widely in the pharmaceutical industry and as food additives, and the production of either the natural or nonnatural amino acids is at least one advantage shown by the organometallic catalysts compared to enzyme systems. The incorporation of an effective rhodium catalyst into a protein begins to bring closer together analogies between the two areas. Interest is growing in chiral catalysts based on less expensive metals, such as cobalt, and a wider range of chiral ligands–including naturally occurring ones, is being exploited.