A conceptual design and supporting analyses are presented for an in situ, on-orbit particulate-contamination monitor for the AXAF High Resolution Mirror Assembly (HRMA), a six-shell Wolter-Type-I grazing incidence telescope. The monitor technique is based on the measurement of light scattered perpendicularly from the mirror surface from a near-IR laser beam inserted into the HRMA optics spirally at grazing incidence. Results of a proof of concept experiment indicate sensitivity of scatter to the fraction of a mirror surface obscured by particles (CF) to be 0.077/sr/CF, compared with a theoretical prediction of 0.096/sr/CF. A noise-equivalent CF (NECF) of 6.2 x 10 to the -7th limited by area sampling statistics was shown to be achievable in the laboratory configuration, from which an NECF below 5 x 10 to the -7th is projected for a flight implementation of the technique. Surface illumination efficiencies of 0.63 and 0.51 for the HRMA inner paraboloid and inner hyperboloid, respectively, are shown to be achievable. The monitor design employs commonly available space qualified optical components, is low in cost, and will afford continuous monitoring of HRMA contamination levels throughout the AXAF mission, from assembly on the ground through end of life in space.