Few works in the field of second language acquisition (SLA) can endure multiple reads, but Selinker's (1972) "Interlanguage" is a clear exception. Written at the inception of the field, this paper delineates a disciplinary scope; ask penetrating questions; advances daring hypotheses; and proposes a conceptual and empirical framework that continues to stimulate SLA research. Sparked by a heightened interest in this founding text on its 40th anniversary, 10 leaders in their respective fields of SLA research collectively examine extrapolations of the seminal text for the past, the present, and the future of SLA research.