The new standards for K-12 science education suggest that student learning should be more integrated and should focus on crosscutting concepts and core ideas from the areas of physical science, life science, Earth/space science, and engineering/technology. This paper describes large-scale, urban elementary-focused science, technology, engineering, and mathematics (STEM) collaboration between a large urban school district, various STEM-focused community stakeholders, and a research-focused private university. The collaboration includes the development of an integrated STEM curriculum for grade K-5 with accompanying teacher professional development. This mixed-methodology study describes findings from focus group interviews and a survey of teachers from Title I elementary schools. Findings suggest the importance of the following critical features of professional development: (a) coherence, (b) content focus, (c) active learning, (d) collective participation, and (e) duration to the success of large-scale STEM urban elementary school reform.