Teachability and Learnability across Languages addresses key issues in second, foreign and heritage language acquisition, as well as in language teaching. Focusing on a Processability Theory perspective, it brings together empirical studies of language acquisition, language teaching, and language assessment. For the first time, a research timeline for the role of instruction in language learning is presented, showing how the field of second language acquisition (SLA) research has developed over the last four decades since Pienemann's work on learnability and syllabus construction over the 1980s. The book includes studies of child and adult second as well as foreign language acquisition research, covering a wide range of target languages including English, German, Hungarian, Japanese, Norwegian, Polish, Spanish, Swedish, and Turkish. In addition, future extensions of PT are discussed. This volume is designed for advanced students in international programs of SLA and Applied Linguistics as well as for SLA researchers and second and foreign language teachers.