The present status of the application of Hartreeâ Fock theory to periodic systems using gaussian basis sets is reviewed. The most important technical aspects are discussed in a language close to that of the molecular theoretician, so that the method can be seen as similar to a molecular calculation but with a number of extra features, both simplifying and complicating. The level of agreement currently achieved between theory and experiment for such data as lattice parameters, X-ray structure factors and electron momentum distributions (Compton profiles) is illustrated. The considerable potential for exploiting a number of the ideas stemming from periodic systems in the context of calculations on large molecules is highlighted.