A microscopic theory of the correlation operator, directed toward conduction electrons in a metal, is developed. The off-diagonal matrix elements, between electron states k--> and k-->+q-->, that arise when the electron density has a modulation of wave vector q-->, determine the correlation contribution to band structure. These are computed explicitly. They depend dramatically on k--> (i.e., nonlocal behavior) and on q-->. The sum of exchange and correlation operators is also nonlocal, but is less singular than either individually. An alternative division of exchange plus correlation into screened-exchange plus Coulomb-hole operators is made. It is shown that the (often ignored) Coulomb-hole operator is usually much larger than the screened-exchange operator.