This thesis deals with the physics of two photon processes. We first introducerncovariant perturbation theory and apply it to some quantum electrodynamicsrnprocesses. We start with study of electron-positron pair production in the field of arnstatic nucleus. We then deal with applications to electron-positron pair productionrnthrough collision of virtual photons in high energy electron-electron scattering. Nextrnwe study the pair conversion of bremsstrahlung photon in high energy proton-protonrnscattering. Detailed evaluations of cross sections are given