  We present the photometric calibration of the HST Advanced Camera for Surveys
(ACS). We give here an overview of the performance and calibration of the 2 CCD
cameras, the Wide Field Channel (WFC) and the High Resolution Channel (HRC),
and a description of the best techniques for reducing ACS CCD data.
  On-orbit observations of spectrophotometric standard stars have been used to
revise the pre-launch estimate of the instrument response curves to best match
predicted and observed count rates. Synthetic photometry has been used to
determine zeropoints for all filters in 3 magnitude systems and to derive
interstellar extinction values for the ACS photometric systems.
  Due to the CCD internal scattering of long wavelength photons, the width of
the PSF increases significantly in the near-IR and the aperture correction for
photometry with near-IR filters depends on the spectral energy distribution of
the source. We provide encircled energy curves and a detailed recipe to correct
for the latter effect.
  Transformations between the ACS photometric systems and the UBVRI and WFPC2
systems are presented. In general, two sets of transformations are available: 1
based on the observation of 2 star clusters; the other on synthetic photometry.
We discuss the accuracy of these transformations and their sensitivity to
details of the spectra being transformed.
  Initial signs of detector degradation due to the HST radiative environment
are already visible. We discuss the impact on the data in terms of dark rate
increase, charge transfer inefficiency, and hot pixel population.
