This paper reconsiders the nature of second language grammars—whether a language learner’s ability can be characterized as I-language or E-language (Chomsky 1986)—and how the nature of second language compares to that of first language grammars. It responds to these questions by arguing that language learners—both first and second—develop I-language over time in similar and different ways. The first section reviews conceptions of competence, performance and language variation, particularly in terms of first language acquisition. The next section considers these same points in terms of less systematic systems, those of atypical first language development (Specific Language Impairment) and second language acquisition. The third section discusses recent work on endstate interlanguage grammars that will elucidate the nature of second language deficits and the contributing causes for them.