Hydrogen (H2) is the simplest diatomic molecule. Despite this simplicity, H2 plays a very significant role in today's economy and is destined to become even more important in the future. Hydrogenation—the chemical addition of H2 across double or triple bonds, mediated by transition-metal catalysts—is used in the production of a wide range of products, including foodstuffs, plastics, rubbers, commodity and specialty chemicals, and pharmaceuticals. Research efforts targeting more efficient hydrogenation processes have been prompted by concerns over the environmental and health impact of residual metals as well as the high cost of precious-metal catalysts. Perhaps more familiar is the impending role for H2 as a major energy source. This prospect promises the environmental “holy grail” for a fuel whose consumption generates only water. For this to be implemented on a large scale, the scientific and engineering research communities must find new ways to provide hydrogen on demand, quickly and cheaply, as well as to recharge spent material. Regardless of the application, the development of new technologies for storing or using hydrogen will require the discovery of fundamentally new H2 chemistry.