In this article we highlight some of the recent developments in the aqueous-phase hydrogenation of alkenes and arenes. The traditional types of water-soluble catalysts applied in aqueous media are outlined together with recent trends towards the development of transition metal nanoparticle and nanocomposite catalysts. In addition, we describe some palladium nanoparticle catalysts that are stabilized by ionic polymers, which demonstrate superior activities in the aqueous-phase catalytic hydrogenation of styrene, used as a model substrate, compared to related palladium nanoparticles. (C) 2014 Elsevier B.V. All rights reserved.