In the first part of this article, empirical evidence is reviewed that suggests a substantial amount of flexibility and context-sensitivity in peopleUs judgments of similarity. Four examples of flexible similarity from our laboratory are considered in detail. In the second part of the article, evidence for relatively constrained, invariant similarity assessments is considered. In the final section, a resolution to these apparently contradictory views on similarity is proposed. Assessments of similarity are used to make inferences from one entity to another. In some situations, flexible similarity is needed to tailor inferences to oneUs knowledge of the entities and their relations. In other situations, particularly those in which specific knowledge is missing or unavailable, a relatively constant similarity is needed to establish generally permissible inferences. Thus, the flexibility and stability of similarity may reflect its different cognitive uses.