New experimental results on self‐sustained pulsations in semiconductor lasers are presented and compared with theoretical predictions. These results show that laser diodes with high external differential quantum efficiency (&eegr;ext) and low coefficient of spontaneous emission coupled into lasing modes (&bgr;) have a significantly higher probability of developing pulsations, which is in good agreement with theoretical predictions. Furthermore, these results provide strong evidence that the laser cavity contains defects which act as optically saturable absorbing centers. These may be incorporated into the active layer of the laser during crystal growth and/or subsequent fabrication.