The degradation of laser diodes constitutes a challenge for laser manufacturers and end users. The catastrophic optical damage (COD) of laser diodes consists of the sudden drop off of the optical power. COD is generally associated with a thermal runaway mechanism in which the active zone of the laser is molten in a positive feedback process. Degraded devices present dark line defects (DLDs) along the laser cavity produced during the laser operation; these DLDs are regions of the active zone of the laser with very low or null light emission as revealed by cathodoluminescence (CL) studies of the degraded regions of the laser. These dark lines are locally generated, either at the front facet, or inside the cavity, propagating along the cavity driven by the optical field. The physical sequence leading to the formation of such lines and the associated loss of output optical power is described in the literature; however, there is a lack of consensus about the connection between the successive steps leading to COD. Understanding this is crucial to improve the technological factors that can strengthen the laser diodes. The full sequence of the degradation consists of different phases, in the first phase a weak zone of the laser is incubated, and the local temperature is increased in such a zone; when a critical temperature is reached the thermal runaway process takes place. Usually, the positive feedback leading to COD is circumscribed to the sequential enhancement of the optical absorption by the increase of the temperature. However, the meaning of the critical temperature has not been unambiguously established. Herein, we will discuss about the critical temperature, and the physical mechanisms involved in this phase; in particular, we will describe the defect morphology and the conditions under which such critical temperature can be reached, both in the front mirror face and the inner cavity. For this, we will analyze the meaning of the critical temperature and the influence of the progressive decrease of the thermal conductivity of the laser structure on the degradation during the laser operation. We will compare the critical temperature estimated by a thermomechanical model with the values usually reported, which range between 130ï¿½C and 200ï¿½C.