Two-dimensional (2D) transition-metal dichalcogenide (TMD) nanosheets have emerged as a fascinating new class of materials for catalysis. These nanosheets are active for several important catalysis reactions including hydrogen evolution from water. The rich chemistry of TMDs combined with numerous strategies that allow tuning of their electronic properties make these materials very attractive for understanding the fundamental principles of electro-and photocatalysis, as well as for developing highly effi cient, renewable, and affordable catalysts for large-scale production of hydrogen. Recent developments are highlighted and important challenges in using TMDs as catalysts are also discussed.