The problem of localization of electrons in extended systems is treated. The solution of the electronic Schrodinger equation is simulated using an iterative procedure. The method predicts whether electrons or excitons are localized or delocalized at T=0 in organic or inorganic crystals. The important parameters are reorganization energy and electronic coupling between sites, which are calculated for the smaller single and double site systems, respectively. The occupation of each site, which is fractional, determines bond orders, which in their turn determine bond lengths and reorganization energy. 'Sharp' metal-insulator transitions follow from our model. Applications are given to organic crystals, graphite, and linear polyenes.