LEDs, which are rapidly replacing the traditional light sources for general illumination, present unique challenges with respect to flicker, due to their different electrical behavior in the circuits in which they are embedded. Since the present-day electrical systems are AC, there is potential for LED output to be time varying, because driver electronics are almost always employed and because driver and mains are not ideal. For flicker, time varying light is not an issue if it is not perceived by humans. The human perception aspect introduces enormous complexities, but large body of psychophysics research over many decades has shed much light on this. There now exist well known and recognized standards which provide for the measure and quantification of flicker. This paper offers a general technical perspective on flicker with discussions of each of these topics. (C) The Author(s) 2019. Published by ECS.