Catastrophic optical damage (COD) is a crucial factor that limits the operation and lifetime of ultra-high power lasers. The understanding of the COD is essential to improve the endurance of this kind of devices. The COD is observed as a process in which the active part of the laser diode is destroyed. As a result, characteristic defects are formed, the so called dark line defects (DLDs). These defects are usually generated at the front facet and propagate inside the cavity. Technologically improved mirror facets make them more resistant, and the COD is more likely to occur inside the cavity, far from the mirror facet. The DLDs are clusters of dislocations and molten and recrystallized regions. The nature of these defects suggests that both temperature and stress play a paramount role in the occurrence of the COD process. The COD arises after many hours of normal operation of the laser; therefore, it concerns lasers without grown-in threading dislocations that may cross the active zone of the laser. The dislocations are generated during the laser operation. The COD is a sudden process, in which the DLDs propagate at a very fast speed, fed by the laser self-absorption. The formation of dislocations is a mechanical process demanding that the stresses reach the onset of plasticity so that a massive generation of dislocations is induced. The origin of the forces generating the dislocations is the existence of very local hot spots in the active parts of the laser, corresponding to laser self-absorbing tiny regions. We discuss here the role of the critical temperature, the thermal conductivity, and the mechanical strength of the laser structure in relation to the COD of high power GaAs based laser diodes.