This study aims to elucidate a common professional learning opportunity in Japan, "instructional circles," which has not yet been studied systematically or described in detail. This study uses the mathematical knowledge for teaching (MKT) framework and research on features of professional learning opportunities that facilitate intended outcomes to describe the nature of professional learning opportunities found in instructional circles. Results showed that instructional circles provided learning opportunities for teachers to enrich all aspects of their MKT. Although the facilitation features recommended in the literature were addressed, instructional circles did so in ways that differ from common practices in the United States. Describing the professional learning opportunities of instructional circles provides an opportunity for mathematics teacher educators to step back from overly familiar practices and look at professional learning from a different perspective. The benefit of taking this reflexive step is to view one's own practice more clearly and gain new insights into designing and conducting professional learning opportunities for teachers. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]