Display device ON/OFF detection methods and apparatus are disclosed. Example display activity detectors disclosed herein are to extract regions from respective ones of captured video frames, the regions corresponding to a depiction of a display of a monitored media device Disclosed example display activity detectors are also to compute a distance metric that is to represent an amount a first one of the regions of a first one of the captured video frames differs from a corresponding second one of the regions of a second one of the captured video frames. Disclosed example display activity detectors are further to compare the distance metric to a threshold to determine whether the monitored media device is ON or OFF.