A survey is given of simple predictive models and operational definitions of the metallic state. The purpose of this work is to highlight the essential features of electron localization versus itinerancy in both crystalline and disordered materials. Particular attention is given to systems that not only extend to the limiting insulating and metallic regimes, but also embrace the transition region between these two canonical states. Examples include the periodic table of the elements, doped semiconductors, metal solutions, expanded (supercritical) metals, transition-metal oxides, and small metallic particles or clusters. The necessary and sufficient conditions for metallic character in these systems are reviewed in terms of the Mott, Hubbard and Anderson views of the metallization process.