Prof. Yi Xie’s group of the University of Science and Technology of China reported dramatically enhanced HER catalysis (an electrocatalytic current density of 10 mA/cm2 at a low overpotential of ?187 mV vs RHE and a Tafel slope of 43 mV/decade) from metallic nanosheets of 1T-MoS2 chemically exfoliated via lithium intercalation from semiconducting 2H-MoS2 nanostructures grown directly on graphite. Structural characterization and electrochemical studies confirmed that the nanosheets of the metallic MoS2 polymorph exhibit facile electrode kinetics and low-loss electrical transport and possess a proliferated density of catalytic active sites. These distinct and previously unexploited features of 1T-MoS2 make these metallic nanosheets a highly competitive earth-abundant HER catalyst. Furthermore, this represents the first application of the metallic 1T polymorph of layered metal chalcogenides in catalysis, and this general approach for controlling nanostructures and polymorphism can be useful in modifying many 2D layered materials to enhance their applications in heterogeneous catalysis, solar energy, and high-performance electronics. (J. Am. Chem. Soc., 2013, 135, 17881)