The Math Teachers' Circles project (MTC) is connecting mathematicians and mathematics teachers in middle schools by offering summer workshops and continued communication throughout the year. The workshops focus on mathematical problem solving and include activities that offer multiple entry points. The goal of the workshops is to increase teachers' knowledge of mathematics for teaching and to help teachers use their knowledge to improve student learning of mathematics. In addition to conducting workshops, researchers are investigating what mathematics teachers learn by participating in the workshops and how teachers use what they have learned in their mathematics teaching. The American Institute of Mathematics (AIM) is facilitating Math Circles in 26 states with research sites in Albuquerque, Denver and San Francisco Bay area. Their research questions include: (1) How is the MTC model being implemented at local sites? (2) What are the effects of participation in a MTC on Teachers' Mathematical Knowledge for Teaching? (3) What is the impact of MTC involvement on teachers' approaches toward mathematics and classroom practice? Twelve case studies, based on classroom observations, are offering insights into how teachers use their mathematical knowledge in planning, implementing, assessing, and reflecting on their instruction. Math Teachers' Circle leaders and participants are connected by a digital network organized by AIM. Workshops are offered for mathematicians who would like to be leaders and organizers of local Math Teachers' Circles, and help is provided to local Circles. The purpose of the local workshops is to develop teachers' content knowledge, problem-solving skills, and mathematical habits of mind. The Math Circles supplement other professional development efforts that focus on pedagogy. The MTC model includes five criteria: content focus, active learning, coherence, approximately 50 hours of professional development, and collective participation. Participants are expected to continue to work within the networked community to develop their mathematical knowledge. The research effort is measuring teachers' mathematical knowledge and conducting case studies to investigate the impact of the MTC on mathematics teaching. They are videotaping lessons and using the Mathematical Quality of Instruction observation protocol. The project evaluator is from Colorado State University.