Math in Common® (MiC) is a five-year initiative that supports a formal network of 10 California school districts as they implement the Common Core State Standards in Mathematics (CCSS-M) across grades K-8. This research brief explores how best to select or develop and use classroom observation systems in order to document instructional shifts and inform MiC school district improvement efforts. The report is organized into three main sections: (1) An exploration of what the research literature says about existing observation systems and several design considerations for successful observation systems; (2) A detailed discussion of several considerations of these findings for school districts as they implement observation systems in order to better track and understand how teachers are implementing the Common Core State Standards in Mathematics in their classrooms; (3) An annotated bibliography for recent publications on classroom observations that might be of interest to school districts interested in exploring these ideas in more depth. The following are appended: (1) Methods for Annotated Bibliography; and (2) Classroom Observation Analysis Tool. [For Volume 1, "Under Construction: Benchmark Assessments and Common Core Math Implementation in Grades K-8. Formative Evaluation Cycle Report for the Math in Common Initiative, Volume 1," see ED559581.]