This chapter focuses on the relaxation of semiconductors from excited states (e.g., obtained by photoexcitation as described in Chap. 8 ) back to the ground state, i.e., recombination. It introduces radiative recombination as well as several possible mechanisms of nonradiative recombination (including Auger recombination and Shockley-Read-Hall recombination), both qualitatively and quantitatively. Examples are given for the recombination rates and lifetimes in both intrinsic and doped semiconductors. Finally, a framework is developed to describe the competition between different recombination processes, followed by a presentation of measurements that can quantify this competition in bulk materials and in single nanostructures.