This paper summarizes a simple single-mode theory of a semiconductor laser and two kinds of multimode extensions. The theories are based on an quasi-equilibrium Fermi-Dirac model of a two-band semiconductor laser gain medium. We include cavity boundary conditions and find the laser single-mode steady-state oscillation intensity. The question as to when sidemodes can build up leads to consideration of a theory of multiwave mixing in the semiconductor medium. This theory is also useful in saturation spectroscopy and phase conjugation using such media, but it does not predict the saturation behavior of the sidemodes. We mention a third-order multimode theory of the laser that allows for sidemode saturation and includes the many-body effects of band-gap renormalization and Coulomb enhancement. These multimode theories assume that the intermode beat frequencies are small compared to the carrier-carrier scattering rate, an assumption that should be valid for external-mirror semiconductor lasers. Using a simple model for the beat frequencies comparable to the carrier-carrier scattering rate, we find two-level inhomogeously broadened sidemode gain and coupling coefficients. Population pulsations and spectral hole burning play approximately equal roles in this theory.