The question of "what makes things seem similar?" is important both because of similarity's pivotal role in theorics of cognition and because of an intrinsic interest in how people make comparisons. In many cases similarity seems to involve more than listing the features of the things to be compared and comparing the lists for overlap. Instead, when comparing things that are hierarchically or propositionally structured, the parts of one thing must be aligned. or placed in correspondence, with the other thing's parts Three quantitative models of similarity are developed and applied to the similarity ratings from two experiments. The model with the best overall fit assumes an interactive activation process whereby correspondences between the parts of compared things mutually and concurrently influence each other. An essential aspect of this model is that matching and mismatching features influence similarity more if they belong to parts that are placed in correspondence. In turn, parts are placed in correspondence if they many features in common and if they are consistent with other developing correspondences. Three predictions of the model are empirically confirmed.