Many school systems are adopting new curricula in response to more rigorous standards that require higher-order thinking skills. This chapter presents implementation findings from a randomized, controlled trial of the Cognitive Tutor Geometry curriculum. We found a significant negative effect on student achievement despite the curriculum's focus on learner-centered learning strategies that have previously been found to improve students' ability to meet high mathematics standards. Our research confirms prior research that finds learner-centered instructional practices are correlated with higher student achievement. However, our findings also suggest that learner-centered curricula can actually do more harm than good when implemented poorly. We found that the cognitive demands of the curriculum coupled with teachers' poor implementation of learner-centered instructional practices seemed to limit students' ability to engage with the mathematical ideas. Teachers struggled to implement the curriculum because they lacked prior experience with learner-centered teaching strategies, had limited exposure to the curriculum, and were not provided with job-embedded support from principals or instructional leaders within their school. They also worked with students who were reluctant to collaborate and had low prior math achievement. Findings from this study suggest that curriculum adopters should be careful to ensure strong implementation of cognitively demanding curricula. In particular, districts and school leaders should provide intensive job-embedded professional development and support to assist teachers in achieving high implementation.