Performance of the cesium iodide calorimeter at the KTeV experiment at Fermilab

We describe the KTeV pure cesium iodide calorimeter and explain the calibration of the detector. The calorimeter has a global energy resolution better than 0.75% and an energy non-linearity less than 0.5% between 3 and 75 GeV. We discuss the performance of the calorimeter during the data-taking runs in 1996 and 1997 and briefly comment on the performance in 1999.