An experimental investigation of discomfort due to flicker in lighting installations has been conducted. Most of the experiments were on a full scale with the whole field of view undergoing fluctuation of illumination. The work has shown that the flicker problem in well-engineered installations is not a serious one, and only affects a small proportion of the population. Wide variations in flicker sensitivity have been found, and it has been established that small differences in frequency produce large differences in flicker sensation. Flicker may be more readily perceived in a large area of moderate luminance, for example a white drawing board, than in a small area of high luminance, for example a bare fluorescent lamp. Apart from obviously faulty auxiliaries or lamps, the most likely cause of flicker in fluorescent discharge lamp installations fed from a 50-cycle A.C. supply is the presence of a 50-cycle component superimposed on the normal 100-cycle wave-form.