Mossbauer spectroscopy is a widely used analytic too] which provides information about local electronic Structure of solid materials on an atomic scale. The isomer shift of resonance nuclear gamma transition is a sensitive parameter which depends on the charge and spin state of the resonating atom as well as on its chemical environment. Theory underlying the isomer shift is reviewed and its connection to the local electronic structure is discussed. A review of advances made in the ob initio calculation of isomer shift is presented. The importance of Careful calibration of the parameters of nuclear gamma transitions on the basis of high-level quantum chemical calculations With the inclusion of both relativistic effects and electron correlati...