Lightwave communication systems employing junction lasers as sources are currently being evaluated in realistic field environments around the World. In many respects, a laser diode is an ideal source for these systems since it emits adequate power for economical fiberguide transmission and is compatible with fibers integrated optics, and integrated circuits, efficient in its conversion of electrical to optical power, mechanically rugged, potentially reliable, and easily modulated in a variety of ways. This paper reviews the modulation characteristics of contemporary cw junction lasers with a view to identifying current limitations on their performance. We focus on the (AlGa)As double-hereto-structure (DH) diode fabricated in a stripe-geometry since this device is the most developed for fiberguide systems.