While all languages use shape to classify unfamiliar objects, some languages as diverse as Mandarin, Thai, Japanese, Mohawk, and American Sign Language lexicalize these and other types of description as noun classifiers. Classification does not develop from a fixed set of features in the object, but is discourse-sensitive and invoked when it would add information for the listener. Children and adults develop classifiers in the same order, and four stages of classifier development are seen: (1) no classifiers at all; (2) division of animates and inanimates; (3) shape classification by one, two, and then three dimensions; and (4) conventionalization of functional classifiers. Shape is the most common feature chosen for classification. Classification by shape rather than function is reinforced by informativeness, natural forms, perceptual development, and cross-sensory reinforcement, and it develops out of universal, biologically structured experience rather than purely linguistic functions. Although adults have more sophisticated functional and stylistic categories than children, when either is under communicative stress, both revert to the same simplified pragmatic mode of using common, perceptually-based classifiers that they can count on their listeners to understand. (MSE)