Test-based accountability policy in the US has resulted in increased availability and use of assessment systems to support instructional decision making. Though limited, literature to date suggests an important role for these systems as tools, mediating teacher knowledge and action. We examine system log data from NWEA's MAP assessment to better understand how and when educators use the system, report features valued by educators, and patterns of use at the individual, school, and district levels. Findings reveal limited engagement with the system, occurring primarily around testing windows with a small subset of reports. We describe the features of those reports as well as how those features relate to use across the school year. We also find use is primarily an individualized practice, with little variability in use accounted for among schools and districts. We conclude with implications for research as well as local policy and practice.