Calculations on encapsulation of a metal atom in silicon clusters have led to the predictions of novel forms of nanosilicon and the discovery of silicon fullerenes. There is renewed interest in the study of silicon nanoclusters and the understanding of the effects of doping with metal atoms. Experiments on doped clusters of silicon as well as other group 14 elements have led to the realization of some of the predictions. Encapsulation of a metal atom enhances the stability of nanoclusters and often leads to a striking preference for a specific size. This can facilitate the design and production of nanoclusters of silicon with specific properties in high abundance and the development of assemblies of such species. This chapter discusses the electronic origin of the stability of such species as well as their magnetic, vibrational and optical properties. Hydrogen termination has been used to predict empty cage silicon fullerenes as well as their endohedral and exohedral forms that could have potential for the development of many derivatives and functional materials similar to carbon fullerenes as well as their interesting optical and magnetic properties for nanoscale applications. {\textcopyright} 2008 Elsevier Ltd All rights reserved.