The article argues that Norton Peirce's (1995) concept of a language learner's investment should figure centrally in how instructors address the needs of adult learners in ESL classrooms. Investment is discussed in relation to second language acquisition research that addresses the role of social factors in second language acquisition. The article then expands on the nature of the relationship between an instructor and students in terms of engaging and sustaining investment in the target language. Finally, the article suggests directions for further research in applying the concept of investment to classroom practice. (Contains 2 notes.)