The study of equivalence relations exhibited by individuals with mental retardation and language limitations holds the promise of providing information of both theoretical and practical significance. We reviewed the equivalence literature with this population, defined in terms of subjects having moderate, severe, or profound mental retardation. The literature includes 55 such individuals, most of whom showed positive outcomes on equivalence tests. The results to date suggest that naming skills are not necessary for positive equivalence test outcomes. Thus far, however, relatively few subjects with minimal language have been studied. Moreover, we suggest that the scientific contributions of studies in this area would be enhanced with better documentation of language skills and other subject characteristics. With recent advances in laboratory procedures for establishing the baseline performances necessary for equivalence tests, this research area is poised for rapid growth.Key words: stimulus equivalence, mental retardation, language limitations, relational learning, conditional discrimination, humansThe 30th anniversary of the publication of Sidman's first paper on equivalence relations was marked in 2001. The study (Sidman, 1971) demonstrated rudimentary reading comprehension of 20 words in an adult male with severe mental retardation (MR). At the beginning of the study, the man could neither match the printed words to representational pictures nor select the printed words upon hearing them spoken. He could, however, select the corresponding pictures upon hearing most of the words spoken, and he was taught to select the pictures that he did not initially know. He also could correctly name most of the pictures before any training. He was taught to select the corresponding printed word upon hearing each of the 20 words spoken. After mastering the spokento-printed word task, the subject then selected printed words to match their corresponding pictures, and vice versa. Further, he named most of the printed words. This outcome demonstrated that the man learned more about the stimuli than was taught di- rectly. Emergent performances like these are critical components of language development.Sidman and Tailby (1982) formalized a set of tests designed to distinguish conditional relations that were merely unidirectional, rote chains from those that could serve as prerequisites of emergent relations. They called the latter ''equivalence relations.'' Sidman and colleagues proposed that the definition of equivalence relations captured essential properties of the kind of relational learning that is typically labeled as symbolic or representational. For example, consider a child who learns to select the numeral 2 (from among several numerals) when he hears the word ''two,'' and also learns to select the printed word two (from among several words) when he sees the numeral 2. In a positive test for reflexivity, the child selects the numeral 2 when shown the numeral 2. In a test for symmetry, the printed word is presented as the...