This article discusses the several contributions to the magnetic moment of an ion in a crystal and how such individual magnetic moments contribute to the total moment of the crystal as a whole. It deals particularly with Co2+ ions in an insulating host, taking MgF2as an example. when two Co2+ ions are in close proximity, exchange effects occur and give rise to a change in the magnetic moment of each ion in the pair. This is normally described as ag-shift and its implications in concentrated magnetic material are discussed.