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We review the theory necessary to describe the class of semiconductor lasers with nonplanar mirrors whose resonator geometry is of the unstable resonator configuration. We present formulas from geometrical unstable resonator theory and calculate quantities relevant to semiconductor lasers, including quantum efficiency and the width limitation due to total internal reflection. We describe the mirror coupled mode formalism for analysing the modes of unstable res onator semiconductor lasers and present near-field patterns and calculations of loss as a function of length, width, and lateral index step. We show that the periodicity of losses as a function of length is determined by a quantity analogous to the equivalent Fresnel number of macroscopic unstable resonators, which is determined by the lateral index step within the laser.