The Responsive Classroom Efficacy Study (RCES) is a randomized controlled trial examining the efficacy of the Responsive Classroom® Approach. The Responsive Classroom® Approach, an intervention developed by the Northeast Foundation for Children, is designed to integrate social and academic learning and improve the capacity of teachers to manage their classrooms effectively. RCES involves 24 schools and data collection began in January, 2008. It is designed as a three year longitudinal study; children and their teachers will be followed from grades three through five with a special emphasis on the extent to which the Responsive Classroom® Approach relates to classroom quality during mathematics instruction and ultimately, children’s math achievement. The M-Scan measure of Mathematics Instructional Quality was developed as part of this research effort. This work is funded by a grant from the U.S. Department of Education, Institute for Education Sciences.