Remotely sensed imagery provides the leading source of land cover data for mapping and monitoring the Earth. Effective use of these data requires that the thematic accuracy of these maps be assessed. Thematic map accuracy assessment is a complex process involving many factors and considerations. The most expensive and time-consuming component of this assessment is the collection of appropriate reference data. Recent developments in Unmanned Aerial Systems (UAS) technologies provides for their effective use for collecting such reference data. This chapter reviews what is required to conduct a thematic accuracy assessment, discusses how UAS may be used to collect reference data, and reports on a pilot study undertaken to evaluate such a collection.