To investigate the breadth of visual illusions experienced by aviators flying with night vision devices (NVDs), an open-ended questionnaire was distributed to the military helicopter community. Of the 242 returned questionnaires, there were 221 image intensification (I2) reports and 21 thermal imaging system reports. Most sensory events occurred at night, during low illumination, good weather, and over varied terrain. Contributing factors included inexperience, division of attention, and fatigue. Frequently reported illusions were misjudgments of drift, clearance, height above the terrain, and attitude. Also reported were illusions due to external lights and disturbed depth perception caused by differences in brightness between I2 tubes. Other respondents cited hardware problems and physiological effects. There were no obvious differences between the experiences of I2 users and FLIR (forward-looking infrared) users. Although incidence rates cannot be inferred from these data, the variety of aviator reports will be useful to all those connected with the human factors and safety of NVDs.