This research studied the calibration of the thermal band (band 6) of the Landsat 5 satellite. The Landsat system of instruments are land observing satellites intended for long-term studies of the earth. Landsat 5 has been in use since 1984 and is still active. Until recently there has not been a rigorous study of the performance of its thermal band. The users of this thermal data had been using the data without knowing the status of the instrument. Over the years the instrument has had the potential to drift in calibration, especially since the instrument has surpassed its expected lifetime. In fact, when Landsat 7 was launched in 1999 there had not been a systematic check of thermal calibration of Landsat 5 since shortly after its launch. Taking advantage of the fact that the two sensors, Landsat 5 and 7, were simultaneously operational, a cross-calibration was possible. As Landsat 7 maneuvered into its orbit, it passed under Landsat 5's path resulting in both sensors having common spatial and temporal coverage. Using this common data the calibration of Landsat 5 was shown to be within measurement error. A historical calibration was also performed on Landsat 5's thermal band. This was done using various Landsat 5 images of the Great Lakes from when it was launched in 1984 to 1999. Early spring images of the Great Lakes were used because of the thermal bar phenomena that occurs. The thermal bar creates a boundary of known temperature with-in the center of the lake. This known temperature boundary is used as a calibration reference. This historical calibration revealed that Landsat 5's thermal band has been operating nominally, within the uncertainties of this approach, over its lifetime.