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; asks penetrating questions; advances daring hypotheses; and proposes a first-ever 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. This book offers a rare resource for novices and experts alike in and beyond the field of SLA.