Laser performance degradation at elevated temperature often requires the use of costly cooling devices. Much effort has been devoted to understand and overcome the high-temperature failure of laser diodes used in telecommunication applications (wavelength 1.3-1.6micrometers ). Various physical mechanisms have been proposed to explain high-temperature effects, including Auger recombination, carrier leakage, intervalence-band absorption, gain reduction and others. The discussion of the dominating effects is still controversial. One reason for this controversy is the use of simplified theoretical models that emphasize selected mechanisms. One-sided models lead to one-sided interpretations of measurements. In this paper, high- temperature measurements on InP laser diodes are analyzed using a comprehensive laser model that includes all relevant physical mechanisms self-consistently. The software combines two-dimensional carrier transport, heat flux, strained quantum well gain computation, and optical wave guiding with a longitudinal mode solver. Careful adjustment of material parameters leads to an excellent agreement between simulation and measurements at all temperatures. At lower temperatures, Auger recombination controls the threshold current. At high temperatures, vertical electron leakage from the separate confinement layer is the main cause of performance degradation. The increase of internal absorption is less important. However, all these carrier and photon loss enhancements with higher temperature are mainly triggered by the reduction of the optical gain due to Fermi spreading of carriers.