Establishing professional communities of educators from districts and schools has gained popularity as a mechanism for collaboratively thinking through and enacting change in education. In 2013, 10 California school districts received grant funding from the S. D. Bechtel, Jr. Foundation ("the Foundation") to participate in the Math in Common (MiC) initiative to support implementation of the Common Core State Standards for Mathematics. The districts were located across the state, ranging in size and characteristics, from large urban districts to small rural districts. A portion of the grant funding to the districts was dedicated to enabling their participation in a community of practice (CoP) to "share lessons learned with each other [about implementing the CCSS-M] and participate in learning communities to share instructional materials and best practices" (S. D. Bechtel, Jr. Foundation, 2012). This report describes the trajectory of the MiC CoP, including both its missteps and its successes, in the hope that designers or participants of other formal or informal CoPs can leverage the MiC experience and make progress more quickly in their own collaborations.