Ground-reference techniques for the Enhanced Thematic Mapper Plus (ETM+) on Landsat 7 are described. The techniques are similar to those used for many years for Landsat-5 Thematic Mapper (TM). Recent results with the Landsat-5 TM are presented, including comparisons with hyperspectral, airborne imaging data. These results show that the Landsat sensor has remained stable within the 5% uncertainty of the ground- reference methods for the last five years. The airborne imagery is also used to show uncertainties due to registration errors, spectral differences, and spatial resolution differences in cross-comparison techniques planned for Earth Observing System sensors. In addition to the use of the traditional methods and test sites, a smaller test site local to the University of Arizona area is being evaluated for use with ETM+. This site, while not as bright, spatially- uniform and large as typical sites, allows more frequent calibrations and hopefully a better understanding of the calibration as a function of time. The selection of the test site, its properties, and example results of calibration of Landsat-5 TM are presented.