This study presents a strategic review of the current quality and effectiveness of more than 40 online professional development (OPD) sites for teachers of mathematics. The evidence gathered about this relatively new form of professional development was compared with evidence on the quality and effectiveness of traditional face-to-face professional development. Individual sites incorporate features of OPD that collectively demonstrate the potential advantages of this new form of professional training. These features include OPD's convenience and accessibility; its capacity to tailor instruction to the individual learner; its video-based multimedia repository of best practices; and its ability to provide uniform, sustained, and mathematically rich, content-based training that is often missing from traditional teacher workshops. This strategic review identified several serious limitations of current OPD sites, but also observed promising features that illustrate the potential for providing high-quality OPD on a large scale. (Contains 1 figure, 4 tables, and 4 exhibits.) [This document was written with the assistance of Linda Rosen and Geneise Cook.]