A target detection system is provided for examining picture signals of a scene which may contain a target signal (15,16) to be detected against background signals (17,18). The image is divided into sub-images and sub-stores are provided one each for the sub-images (19). The values of a set of features, such as brightness, volume or quietness, of the background and target picture signals associated with each sub-image in the respective sub-store are calculated and stored. The statistical distributions of the background and target features in each sub-store to set a target decision threshold, in feature terms, for each sub-image are calculated. The feature values of an incoming picture signal in a sub-image are compared with the decision threshold in the associated sub-store to provide a target detection signal. Each sub-image threshold relates to the image status in that particular sub-image. A lower false alarm rate with a higher detection probability is provided.