Semiconductor injection lasers are key components in fiber optic communication systems, therefore their reliability is a determining factor in the technical success of these systems. Lasers are subject to several specific degradation mechanisms some of which affect the bulk of the optically active volume of the device, some the mirror facets and some the electrical or thermal contacts. These mechanisms are driven primarily by the operating temperature of the device or by the locally produced electrical and optical power density, but their severity is strongly linked to technological factors such as the quality of the crystal growth (substrate and epitaxial layers) and subsequent processing technologies (mirror passivation, mounting, con-tacting, etc.). They also depend on the chosen materials (GaAs/A1GaAs or InP/InGaAsP), that is the emission wavelength, contact materials and mirror passivating materials. The best way to assure high reliability of these devices in the field - in addition to perfecting and controling the manufacturing processes - is the development of accelerated aging tests and the selection of strict and meaningful criteria for the acceptance or rejection of a particular device. Such tests and criteria have been developed supported by experiments and theoretical predictions. It can be shown that carefully manufactured lasers do not degrade more rapidly than LED-s and their mean time to failure at normal operating conditions may even exceed those of other active components of the system such as driver electronics, etc.