This article reviews the status of similarity as an explanatory construct with a focus on similarity judgements. For similarity to be a useful construct, one must be able to specify the ways or respects in which two things are similar. One solution to this problem is to restruct the notion of similarity to hard-wired perceptual processes. It is argued that this view is too narrow and limiting. Instead, it is proposed that an important source of constraints derives from the similarity comparison process itself. Both new experiments and other evidence are described that support the idea that respects are determined by processes internal to comparisons.