One of the basic problems of perturbation and spectral theory is to determine the spectral properties of a perturbed operator H when the spectral properties of the associated unpertubed operator H0 are known. Spectral shift function (SSF), which describes the shift of the eigenvalues of H relative to the eigenvalues of H0, is one of the major tools in describing the relationship between certain traces associated with the two operators. The aim is to introduce spectral shift function and discuss its applications to Schrodinger operators.