A review is presented of theoretical models for describing and rationalizing the development of metallic properties in clusters, as a function of cluster size. Clusters, which span a wide size range, can be approached from the bulk solid (infinite cluster) limit—via quasi–continuum models derived from condensed matter physics—and from the small molecule limit—via discrete quantum mechanical models. Smooth and oscillatory cluster size effects are described and their origins rationalized. The competition between electronic and geometric shell stability effects, and the importance of the cluster temperature and its physical state, are discussed. The archetypical metal to non–metal transition observed in mercury clusters, as a function of size, is considered and the possibility of forming metallic clusters from non–metallic elements is investigated.