Catalytic hydrogenation reactions are ubiquitous in the industrial synthesis of both bulk and fine chemicals. The addition of H2 to a substrate molecule is atom economic and generally costeffective, making such reactions very attractive from an environmental point of view. This prospect has motivated a continuous effort towards understanding the activation of H2 by transition metal catalysts. Classically, reduced transition metal centers activate the H-H bond via two bonding interactions (Figure , a): s-donation by the H2 ligand to a metal vacant site and πbackdonation from the metal d-electrons to the s*(H-H) orbital. Strong orbital interactions result in oxidative addition, fully cleaving the H-H bond to form a metal dihydride (Figure ). In contrast, weaker interactions result in non-classical dihydrogen complexes, in which some extent of H-H bonding is preserved. Activation of molecular H2 with 3d metals is generally more challenging than with 4d or 5d metals because they tend to form weaker M-H bonds. Nevertheless, the use of 3d metals for hydrogenation reactions has attracted considerable interest in view of their abundance, low cost, and generally lower toxicity. Bifunctional catalysts that rely on ligands actively participating in the cleavage of the H-H bond play a central role in this transition (Figure ). In particular, Lewis bases incorporated in the ligand can accept a proton from the H2 molecule to generate a hydride. A related, very successful strategy relies on reversible aromatization/dearomatization of a N-heterocyclic ligand. In both of these processes, the formal oxidation state of the metal remains unchanged. More recently, Lewis acids have been found able to accept a formal hydride from H2 to form a M-H bond with concomitant oxidation of the metal. Additionally, bimetallic systems have been successfully applied, in which metalmetal proximity helps polarizing and cleaving the H-H bond. Figure . Strategies of H2 activation in homogeneous catalysis.