Context: This study analyzes the implementation of a blended learning middle school mathematics intervention in a large urban school district in the northeastern United States. Blended learning models integrate online instructional tools within traditional methods of classroom instruction. Focus of Study: As their use increases in classrooms across the country, there remains much unknown about how teachers integrate blended learning strategies into their pedagogical practices and what factors, including school, teacher, and student attributes, facilitate or hinder these approaches. Our findings provide insight into how teachers integrate computer-based curricula in their classrooms, findings particularly instructive for under-resourced urban school districts. Research Design: The study is designed as part of a within-teacher randomized controlled field trial, a design which enables direct comparisons of teacher practices between each of his or her two mathematics classrooms. To draw these comparisons, we utilize a range of detailed teacher survey data as well as rich teacher interview data. Conclusions: We believe our findings about the choices teachers make in using software in the classroom and the barriers they face in doing so are applicable to the implementation of a wide variety of computer-based interventions in urban environments, whether they are part of curricular innovations, blended learning instructional strategies, or targeted academic interventions.