The stability of semiconductor lasers (SCL) is a function of junction temperature. This temperature dependence can be modelled by solving the time-dependent heat flow equation. In the present study we have first designed the criteria for maximum temperature fluctuations for stable operation of SCL. The temperature fluctuation due to pulsed input is calculated as a function of frequency and duty cycle. The fluctuation is found to be maximum at 50% duty cycle. In these calculations, power dissipation in SCL is assumed to consist of two parts: (a) almost all input power is dissipated as heat up to threshold condition; and (b) above threshold, a fraction of power is extracted as optical output. The effect of confinement layer thickness is also studied. It is found that both the average temperature rises and the fluctuation increases with thickness; whereas the variation of active layer thickness does not affect the fluctuation per watt dissipated power. However, the power dissipated at the threshold and the critical output power are reduced by the same factor as the active region thickness. The transient analysis is done for a broad area laser where current and heat flow are nearly one-dimensional, employing Liebmann's implicit method. However, many lasers intended for use outside the laboratory are of the stripe geometry type. These lasers have two-dimensional current and heat flow. A steady-state analysis is used to simulate the temperature distribution in stripe geometry lasers employing the successive over-relaxation method. The mean thermal resistance and the temperature drop from the centre to the edge of the stripe are calculated as a function of stripe width (5 μm to 50μm) and confinement layer thickness (2 μm to 5 μm). It is found that heating is non-uniform across the stripe, which results in the presence of different longitudinal modes at different portions of the stripe. This leads to non-uniform intensity at the facet and limits these lasers' use in optical communication systems. Non-uniformity of emission also affects the device lifetime.