This major contribution to second language acquisition theory examines the question of the systematicity of learners’ language. Richard Young proposes a new descriptive model for handling what other investigators have claimed to be random variations in performance, and he tests the model on plural inflections in the English interlanguage of Chinese learners. The study investigates how factors such as the social context of speech, the linguistic environment of a variable, and the tendency to omit redundant information affect the developing interlanguage system. The representation of learners’ language which emerges from this study is richer, more complex, and more descriptively adequate than has previously been available.