This monograph is part of a series on second language research methodology edited by Gass and Schachter. Their introduction to the text highlights the interdisciplinary nature of SLA research and the necessity for a fuller understanding of the wide variety of research methodologies in current use. Each monograph is devoted to a particular data collection method or instrument and provides an overview of (a) the theoretical and research background, (b) the current uses of the instrument or method, (c) appropriate research questions, (d) underlying assumptions, and (e) any problems associated with practical use. Forthcoming texts will cover case studies, conversation analysis, elicited imitation, data-driven modeling, and speech acts.