The concept of autonomy in second language learning is explored by tracing some steps in the evolution of theory and practice. The first section relates one definition of learner autonomy ("the ability to take charge of one's learning") to self-instruction in language learning and to research in language learning at the adult level. The second section describes a successful attempt to promote autonomous learning in a secondary school classroom and subsequent developments in theory. The third section then elaborates a theory of learner autonomy designed to apply differently to these two very different kinds of language learning, incorporating the notions of action knowledge, zone of proximal development, and other principles of child development. The fourth describes a current application of the theory to language learning projects at the Centre for Language and Communication Studies at Trinity College (Dublin, Ireland), and a concluding section suggests how the theory of learner autonomy might be further explored and elaborated. Contains 16 references. (Author/MSE)