The thesis analyses the process of pair annihilation into one photon in a laser pulse. The theory of how to include pulse shapes in Strong Field QED and the resulting cross section is presented. The cross section is calculated and estimated for lasers of ELI and XFEL facilites. It is found that the effect may be experimentally verifiable at high frequency XFEL facilities for very finely tuned particle kinematics, but negligible at high intensity optical laser facilities such as ELI.