The Independent Detector Testing Laboratory (IDTL) has been established by the Space Telescope Science Institute (STScI) and the Johns Hopkins University (JHU), and it will assist the Next Generation Space Telescope (NGST) mission in choosing and operating the best near-infrared detectors. The NGST is the centerpiece of the NASA Office of Space Science theme, the Astronomical Search for Origins, and the highest priority astronomy project for the next decade, according to the National Academy of Science. NGST will need to have the sensitivity to see the first light in the Universe to determine how galaxies formed in the web of dark matter that existed when the Universe was in its infancy (z ~10-20). To achieve this goal, the NGST Project must pursue an aggressive technology program and advance infrared detectors to performance levels beyond what is now possible. As part of this program, NASA has selected the IDTL to verify comparative performance between prototype NGST detectors developed by Rockwell Scientific (HgCdTe) and Raytheon (InSb). The IDTL is charged with obtaining an independent assessment of the ability of these two competing technologies to achieve the demanding specifications of the NGST program within the 0.6-5 μm bandpass and in an ultra-low background (<0.01 e-/s/pixel) environment. We describe the NGST Detector Characterization Project that is being performed in the IDTL. In this project, we will measure first-order detector parameters, i.e. dark current, read noise, QE, intra-pixel sensitivity, linearity, as functions of temperature, well size, and operational mode.