Instructional coach initiatives aimed at teachers' professional development are expanding in reforming school districts across the United States. This study addresses the lack of research regarding the professional development of instructional coaches. Drawing on sociocultural learning theory, specifically a model called the Vygotsky Space, the authors use a case approach to examine the learning experiences of a single secondary literacy coach. Hypotheses suggest that (a) coaches are not unproblematic conduits of reform ideas but are also learners of new content and pedagogy; (b) as coaches' conceptual development about instruction grows, their ability to coach also matures; and (c) professional development that supports coaches is best aligned around a workplace pedagogy that addresses the learning needs of multiple system actors. (Contains 1 table, 3 figures, and 8 notes.)