This article reviews the status of similarity as an explanatory constr
uct with a focus on similarity judgments. For similarity to be a usefu
l construct, one must be able to specify the ways or respects in which
two things are similar. One solution to this problem is to restrict t
he notion of similarity to hard-wired perceptual processes. It is argu
ed that this view is too narrow and limiting. Instead, it is proposed
that an important source of constraints derives from the similarity co
mparison process itself. Both new experiments and other evidence are d
escribed that support the idea that respects are determined by process
es internal to comparisons.