The important experimental parameters affecting the mode locking of a variety of (GaAl)As injection lasers operating in an external optical cavity are described. We find that short detector-limited pulses (less than 60 psec) with 100% modulation depth can only be obtained using lasers that exhibit either an anomalous narrowband noise resonance or self pulsations. Little or no mode locking is observed in lasers having the normal broad noise resonance. The maximum frequency of the mode-locked pulses is ≈1 GHz and is limited by the laser and not the external cavity. The observed amplitude, pulse width, and frequency of the mode-locked pulses are correlated to the degree of self-pulsation and the external cavity length. The experimental results are in qualitative agreement with a model that uses the rate equations modified by either electron traps or saturable absorbers and a delayed feedback term. Our results appear to imply that mode locking in (GaAl)As injection lasers is related to saturable absorbing centers and is very similar to passive mode locking in dye lasers.