The theory of recombination via defects having energy levels in the forbidden gap is reviewed. Emphasis is given to those aspects which complicate interpretation of lifetime data, such as the inherent difference between steady state and transient measurements, large‐signal behavior, competing recombination mechanisms, trapping, the possible existence of strongly temperature‐dependent cross sections, and the properties of multilevel defects. A summary of the known recombination properties of bombardment‐produced defects is given.