  Most of the physics analyses at a hadron collider rely on a precise
measurement of the energy of jets in the final state. This requires a precise
in situ calibration of the calorimeter with the final detector setup. We
present the basic procedure and results of the in situ gain calibration of the
D0 HCAL in Run II. The gain calibration works on top of the pulser-based
calibration of the readout electronics and is based entirely on collision data.
