A method of identifying an item of video content, the method comprising the steps of: providing a collection of temporally separated search images, each search image defined by pixel values and each having a temporal location in the collection; for each search image, providing a pair of search hash values comprising: a spatial search hash value comprising a function of a measure of entropy in values of differences between pixel values within the search image, and a temporal search hash value comprising a function of a measure of entropy in values of differences between a pixel value in the search image and a corresponding pixel value in a temporally separated search image; forming search data defining the association between the temporal positions of the search images in the collection and the respective spatial and temporal search hash values; ordering said search data according to the values of a first one of the hash values; for a sought sequence of temporally separated sought images, each defined by pixel values and each having a temporal location in the sought sequence, providing: a spatial sought hash value comprising a function of a measure of the entropy in values of differences between pixel values within a sought image, and a temporal sought hash value comprising a function of a measure of the entropy in values of differences between a pixel value in one sought image and a pixel value in a temporally separated sought image; and searching only the portion of ordered data corresponding to a sought value of the said first one of the pair of hash values to locate the occurrence of a sought value of the second one of the said pair of hash values. providing a collection of temporally separated search images, each search image defined by pixel values and each having a temporal location in the collection; for each search image, providing a pair of search hash values comprising: a spatial search hash value comprising a function of a measure of entropy in values of differences between pixel values within the search image, and a temporal search hash value comprising a function of a measure of entropy in values of differences between a pixel value in the search image and a corresponding pixel value in a temporally separated search image; a spatial search hash value comprising a function of a measure of entropy in values of differences between pixel values within the search image, and a temporal search hash value comprising a function of a measure of entropy in values of differences between a pixel value in the search image and a corresponding pixel value in a temporally separated search image; forming search data defining the association between the temporal positions of the search images in the collection and the respective spatial and temporal search hash values; ordering said search data according to the values of a first one of the hash values; for a sought sequence of temporally separated sought images, each defined by pixel values and each having a temporal location in the sought sequence, providing: a spatial sought hash value comprising a function of a measure of the entropy in values of differences between pixel values within a sought image, and a temporal sought hash value comprising a function of a measure of the entropy in values of differences between a pixel value in one sought image and a pixel value in a temporally separated sought image; and a spatial sought hash value comprising a function of a measure of the entropy in values of differences between pixel values within a sought image, and a temporal sought hash value comprising a function of a measure of the entropy in values of differences between a pixel value in one sought image and a pixel value in a temporally separated sought image; and searching only the portion of ordered data corresponding to a sought value of the said first one of the pair of hash values to locate the occurrence of a sought value of the second one of the said pair of hash values.