This report explores the measurement of high school graduation, completion, and dropout (GCD) rates. First we outline a basic framework for conceptualizing high school completion processes and identifying challenges associated with empirically measuring GCD rates. We then use this framework to construct a catalog of over 70 distinct GCD indicators used by federal and state agencies. The methodological topics addressed here have traditionally been examined by researchers. However, educational accountability systems are now attaching higher stakes to high school graduation rates. This report aims to provide a broader set of stakeholderspolicymakers, educators, the public at largewith a technically-assessable introduction to this important issue.