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 second language acquisition (SLA) research. It is suitable for novices and experts alike in and beyond the field of SLA.

1. Acknowledgements; 2. Introduction (by Han, ZhaoHong); 3. Chapter 1. Enduring questions from the Interlanguage Hypothesis (by Tarone, Elaine); 4. Chapter 2. Rediscovering prediction (by Odlin, Terence); 5. Chapter 3. From Julie to Wes to Alberto: Revisiting the construct of fossilization (by Han, ZhaoHong); 6. Chapter 4. Interlanguage, transfer and fossilization: Beyond second language acquisition (by Montrul, Silvina A.); 7. Chapter 5. The limits of instruction: 40 years after "Interlanguage" (by VanPatten, Bill); 8. Chapter 6. Documenting interlanguage development (by Bardovi-Harlig, Kathleen); 9. Chapter 7. Methodological influences of "Interlanguage" (1972): Data then and data now (by Gass, Susan M.); 10. Chapter 8. Trying out theories on interlanguage: Description and explanation over 40 years of L2 negation research (by Ortega, Lourdes); 11. Chapter 9. Another step to be taken - Rethinking the end point of the interlanguage continuum (by Larsen-Freeman, Diane); 12. Chapter 10. Interlanguage 40 years on: Three themes from here (by Selinker, Larry); 13. Name index; 14. Subject index