Light-emitting diodes are important fibre optic sources not only for short haul, low cost data links, but also in the long haul, high data rate telecommunications field. This paper discusses the range of applicability of l.e.d. sources, and the performance, design and optimization of such devices. Results are given for devices operating both in the low-loss 1.3μm window and in the 0.8–0.9 μm window.