The ROSAT High Resolution Imager (HRI) is very similar to the Einstein Observatory HRI and is comprised of two cascaded microchannel plates (MCPs) with a crossed grid position readout system. The detector performance can be characterized by such parameters as those listed in Table 4.3 of Appendix F. Two significant changes from the Einstein instrument that have affected the performance of the detector are: the substitution of CsI for MgF2 as the detector photocathode, and the introduction of a thin aluminum-coated plastic membrane (the ``electrostatic shield'') near the front MCP (in addition to the UV/Ion shield which is situated farther in front of the detector).