Two important light sources for fiber optic applications, the high radiance etched well LED and the single mode injection laser, are described in terms of their performance characteristics and structure. Requirements for very high power and data rate may dictate the use of lasers over LEDs while low bandwidth links LEDs are more appropriate. When comparing the two devices for the purpose of evaluating their relative suitability in any given F/O systems application, one must fully recognize the wide range of overlap in their performance.