OpenCourseWare (OCW) is one among several models for offering open educational resources (OER). This article explains the OCW model and its position within the broader OER context. OCW primarily represents publication of existing course materials already in use for teaching purposes. OCW projects are most often institutional, carrying the imprimatur of the sponsoring institution and conferring benefits back. MIT launched the first OCW in 2002, and today there are some 150 universities around the world with OCW Web sites running live or in development. Evaluation data show strong user acceptance of the OCW model and demonstrable positive impact for educators and learners.