In this review article we discuss the role of heterostructures in integrated optics. We consider, in sequence, the role of heterostructure waveguides, lasers, light couplers, modulators, and detectors, and describe the progress made to date in integrating these into a thin-film monolithic optical circuit. The primary emphasis is on the GaAs:GaAlAs heterostructure material system, which has emerged as one of the most promising for integrated optics.