Semiconductor laser amplifiers can exhibit either temporal pulse compression or broadening. A transmission-line laser model was used to isolate the mechanisms responsible and to investigate the conditions required for this form of pulse distortion. Results show that gain saturation is the dominant temporal compression mechanism in travelling wave laser amplifiers, and that filtering may account for some compression in Fabry-Perot laser amplifiers. Compression is highly dependent on the input pulse shape, being zero for Gaussian-like pulses. This means that it is unlikely to be useful for shortening pulses produced by highly modulated injection lasers. However, compression can occur with pulses with a slowly decaying trailing edge, but only over a limited range of input powers. Pulse broadening is also caused by gain saturation and is again dependent on the input pulse shape, being zero for square-edged pulses. For Gaussian pulses, the broadening is negligible for input pulse energies below the amplifier's saturation energy. This suggests that TWLAs may be used to boost the power of pulses from mode-locked semiconductor lasers. These results are in excellent agreement with other workers' experimental results.