Metal nanoclusters, which bridge metal atoms and nanocrystals, have become one of the most important types of nanomaterial that have been extensively explored in nanoscience and nanotechnology research in recent years. The unique chemical and physical properties of metal nanoclusters render them potential applications in many fields. Due to the quantum size effect, metal nanoclusters exhibit size-dependent optical and catalytic properties. Therefore, much effort has been focused on studies of the size-controlled synthesis and size-dependent properties of metal nanoclusters. However, in spite of the remarkable success achieved, most synthetic procedures still suffer from the production of a mixture of different cluster sizes. The difficulty in purifying clusters has become a major obstacle to the study of size-related properties and their practical applications. Thus, it is highly desirable to develop novel synthetic strategies that allow for the high-yield synthesis of atomically monodisperse metal clusters. On the other hand, the catalytic application of these atomically monodisperse metal nanoclusters, especially gold clusters, have also been extensively studied. In this chapter, we focus on the recent progress of size-controlled synthesis and the corresponding size-dependent catalytic activity of metal nanoclusters, which are of extreme importance in both the academic and industrial communities.