Object detection apparatus for detecting objects in a test image comprises means for comparing blocks of a test window of the image with reference data indicative of the presence of an object to generate index figures indicative of a degree of similarity between a region and the reference data; means for storing probability data corresponding to possible values of the index figure and the block position; means operable, in respect of a current block, for accessing a probability value from the storing means in dependence on that block's position within the test window and the index figure generated in respect of that block; and means for combining the probability values corresponding to blocks in a test window to generate a result indicative of the probability of that test window containing an object; in which the probability values in the storing means are ordered by block position then by index figure.