  The HAWC collaboration has recently completed the construction of a gamma-ray
observatory at an altitude of 4100 meters on the slope of the Sierra Negra
volcano in the state of Puebla, Mexico. In order to achieve an optimal angular
resolution, energy reconstruction, and cosmic-ray background suppression for
the air showers observed by HAWC, it is crucial to obtain good timing and
charge calibrations of the photosensors in the detector. The HAWC calibration
is based on a laser system which is able to deliver short light pulses to all
the tanks in the array. The light intensity can range over 7 orders of
magnitude, broad enough to cover all the dynamic range of the PMT readout
electronics. In this contribution we will present the HAWC calibration system,
together with the methods used to calibrate the detector.
