Well-defined nano- and atomic-sized heterogeneous catalysts with extremely high catalytic activities and unique selectivities show promise in addressing the critical energy- and environment-related challenges of this century. The exceptional properties of these catalysts, such as their electronic and geometric structures and the effective interactions between metals and supports, give rise to unprecedented catalytic efficiency over that of conventional catalysts. The facile prospects for tuning the active sites of these catalysts pave the way to optimizing their activities, selectivities, and stabilities, thus offering extensive application possibilities in significant industry-related catalytic reactions. A prerequisite for synthesizing nano- and atomic-sized catalyst is to prepare extremely disperse nano- and subnanoscale atoms on suitable supports. This book chapter summarizes various synthesis methods employed to synthesize nano- and atomic-scale catalysts. © 2020 American Chemical Society.