Abstract This chapter gives an overview of the electronic and geometric shell models. These models provide a useful guide to cluster sizes of high stability (magic numbers). The electronic shell model is most applicable to small (<100 atoms) clusters of simple metals. Phenomenological, self-consistent and ellipsoidal versions are discussed, together with its predictions for various metals. The geometric shell model is effective for larger clusters. The Wulff construction is outlined and the various polyhedral forms are described. The mechanisms of filling between complete shells, the phenomenon of supershells and cluster melting are also discussed.