  We provide an overview of the most important calibration aspects of the
NICMOS instrument on board of HST. We describe the performance of the
instrument after the installation of the NICMOS Cooling System, and show that
the behavior of the instrument has become very stable and predictable. We
detail the improvements made to the NICMOS pipeline and outline plans for
future developments. The derivation of the absolute photometric zero-point
calibration is described in detail. Finally, we describe and quantify a newly
discovered count-rate dependent non-linearity in the NICMOS cameras. This new
non-linearity is distinctly different from the total count dependent
non-linearity that is well known for near-infrared detectors. We show that the
non-linearity has a power law behavior, with pixels with high system, or vice
versa, pixels with low count rate detecting slightly less than expected. The
effect has a wavelength dependence with observations at the shortest
wavelengths being the most affected (~0.05-0.1 mag per dex flux change at ~1
micron, 0.03 mag per dex at 1.6 micron).
