In this chapter, the different nonequilibrium phenomena including excitation, recombination, and transport are combined into the continuity equations, which describe their joint effect on the local carrier concentration in a semiconductor. A simplified framework is then developed in the form of the minority carrier diffusion equations. Examples demonstrate how to apply these equations to solve problems involving the combined effects of photogeneration, recombination, and carrier diffusion. The chapter concludes with the definition of quasi-Fermi levels, generalizing the key concept of the Fermi level to semiconductors away from equilibrium.