Similarity and categorization are inextricably linked in cognitive theory. Research on similarity suggests that comparisons involve a process of structural alignment that compares pairs of structured relational representations. Structural alignment is derived from structure-mapping theory, which was originally designed to account for people's ability to process analogies. The structural alignment process focuses people on common relations that hold between the items being compared. This process yields the commonalities of the pair as well as two kinds of differences: alignable differences, which are directly related to the commonalities, and nonalignable differences, which are unrelated to the commonalities. Evidence for this process has been obtained using studies of comparisons of people's existing categories. These data suggest that category representations are structured so that contrasting categories are easy to compare, but more distant categories are difficult to compare. This pattern of data raises the question of how categories come to be structured such that pairs of similar categories are easier to compare than are pairs of dissimilar categories. A possibility explored here is that the category representations are constructed by a reminding-based process that uses elements of representations of categories that are already known.