This chapter describes the activation of dinitrogen by various transition metal hydride complexes. A number of mononuclear transition metal hydride complexes can incorporate dinitrogen, but they are usually difficult to induce N-N bond cleavage. In contrast, multimetallic hydride complexes can split and hydrogenate dinitrogen through cooperation of the multiple metal hydrides. In this transformation, the hydride ligands serve as the source of both electron and proton, thus enabling the cleavage and hydrogenation of dinitrogen without extra reducing agents and proton sources. Generally, the reactivity of the metal hydride complexes is significantly influenced by their composition (nuclearity) and metal/ligand combination.