The need to assess pilot workload during flight has become of increasing importance over the past decade—both in combat aircraft where workload can be excessive, and in civil transport aircraft where underload may occur. Subjective reporting in some form has been the long and well established method for assessing workload in the ‘real world’, but because subjective opinions are vulnerable to bias and to preconceived notions, an additional measure can be of considerable value on occasions. Of die available psychcophysiological variables, recording pilots' heart rates appears to be the most useful. This paper discusses the choice of this measure and presents examples from several studies in which workload was assessed in flight.