1. A computer-implemented method of selecting items to recommend to a user, the method comprising: identifying a plurality of items selected by a user from an electronic repository of items; for each of the plurality of items selected by the user, looking up, from an item-to-item mapping structure, a respective set of related items and, for each related item, a respective data value representing a degree to which the related item is related to the respective item selected by the user, each data value being specific to a particular item pair; generating a combined list of related items that comprises the plurality of sets of related items looked up from the item-to-item mapping structure; sorting the combined list of related items based at least partly on the looked up data values associated with the related items; and selecting, from the sorted, combined list of related items, a subset of items to recommend to the user; said method performed automatically by a computer system that comprises one or more computers.