The general topic of these proposed experiments is stimulus equivalence, a phenomenon in which classes of functionally substitutable stimuli emerge after interrelated conditional discriminations have been trained. Considerable empirical attention has been paid to stimulus equivalence recently because of the impressive generative capacity of these training and testing techniques and their resultant effectiveness in establishing functional skills in developmentally delayed and brain-damaged individuals. Equivalence-based teaching procedures have had a major impact on skills- programming with these special populations. Equivalence class formation is revealed by performances on tests that assess the formal (and logical) mathematical set of properties of reflexivity, symmetry, and transitivity. While human subjects across a wide range of ages have reliably demonstrated the formation of equivalence classes, data from the present investigators' laboratory indicate that class stability is very different for adults and children. Thus, stimulus equivalence procedures may be well-suited to controlled laboratory studies of some of the types of cognitive changes that have played an important role in theoretical treatments of cognitive development. The proposed experiments focus on the extent to which established equivalence classes can accommodate change given new learning experiences and as a function of age. Normally capable children will be individually trained and tested on a computerized match-to-sample task. A cross-sectional study will compare class stability for different age groups when presented with class challenges. A second experiment will determine whether variations in experimental training prior to class challenges can influence class stability, and a third will address possible mechanisms by which language could play a role in equivalence class stability. A final study will provide direct comparison of equivalence class outcomes and ore traditional measures of category learning. These studies may aid in the development of a controlled laboratory model to accompany naturalistic assessment of cognitive level and development and could carry implications for strengthening the application of equivalence procedures.