Catalysis is a multidisciplinary science that serves a broad range of industries covering specialty, fine, intermediate, commodity and life science chemicals. Catalysts are commonly used for the hydrogenation of alkenes, alkynes, aromatics, aldehydes, ketones, esters, carboxylic acids, nitro groups, nitriles and imines. These materials maybe in the form of bio-, homogeneous, heterogeneous and heterogenised homogeneous catalysts where each type has its own special properties that can be adjusted for their optimal use. Heterogeneous catalysts are commonly used in the form of powders for slurry and fluidized bed reactions or as formed bodies for fixed bed hydrogenations. The addition of promoters and adjustments in particle size and porosity allow these catalysts to be fine tuned for specific reactions. Homogeneous catalysts are also very flexible where the choice of active metal, ligands and reaction conditions can lead to highly selective hydrogenations. The separation problems associated with homogeneous catalysts have led to the development of heterogenised homogeneous catalysts via the fixation of the active complexes on organic or inorganic supports or via application in biphasic systems. While there have been some successes in this area, there still remains a considerable amount of work to be done. Biocatalysts are of particular interest for the formation of enantiopure compounds. New developments in cofactor regeneration and raised substrate tolerance help to implement enzymatic reduction reaction on industrial scale. Efficient reductases are still needed to broaden this technology for further applications. It is expected that the development of these catalysts will be accelerated through the use of high throughput screening and that the testing of catalyst libraries will allow for the rapid selection of both catalyst and conditions for targeted processes. Reactor technology will also change as continuous processes slowly replace the workhorse batch ones for the manufacture of large-scale chemicals. This paper provides an overview of the above-mentioned topics for the industrially relevant hydrogenations that have been brought on stream over the last 20 years, as well as, the upcoming processes that will set the future trends in chemical industry. (C) 2004 Elsevier B.V. All rights reserved.