  Schrieffer-Wolff transformation is one of the very important transformations
in the study of quantum many body physics. It is used to arrive at the low
energy effective hamiltonian of Quantum many-body hamiltonians, which are not
generally analytically tractable. In this paper we give a pedagogical review of
this transformation for Anderson Impurity model(SIAM) and its lattice
generlaization called, the Periodic Anderson Model(PAM).
