This book describes the theory of relativistic (e, 2e) processes using one photon exchange approximation on different geometrical arrangements. Using S-matrix approach, triple differential cross section (TDCS) has been derived in terms of atomic transition charge density, atomic transition current density and interference between them. TDCS is also derived in terms of longitudinal part, transverse part and interference between them. The explicit expressions are derived for spin asymmetries using transversely and longitudinally polarized incident electrons on unpolarized target atoms. Dirac spinors with positive and negative helicity are discussed in detail. The double differential cross section (DDCS) has been expressed in terms of longitudinal and transverse contributions and shown that the interference between them is zero which verifies Fano theorem. Electron kinematic factors are expressed explicitly for transversely and longitudinally polarized incident electrons.