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Many websites now warehouse instructional materials designed to be taught by teachers in a traditional classroom. What are the potential benefits of the new resources? We analyze an experiment in which we randomly give middle school math teachers access to existing high-quality, off-the-shelf lessons, and in some cases, support to promote their use. Teachers receiving access alone increased students’ math achievement by a marginally significant 0.06 of a standard deviation. Teachers who received access and support increased students’ math achievement by 0.09 of a standard deviation. Weaker teachers experience larger gains, suggesting that these lessons substitute for teacher skill or efforts. The online materials are more scalable and cost effective than most policies aimed at improving teacher quality, suggesting that, if search costs can be overcome, there is a real benefit to making high-quality …