The large optical apertures required by many space-based telescopes make thermal control of these optics a significant challenge. One technique which has been used for x-ray telescopes involves placing insulating tubes forward of the entrance aperture. The reduction in both conduction and direct view produces a thermal gradient along the tubes, increasing the effective sink temperature for the optics and reducing the effective radiant source temperature and heat flow to space. In another configuration the “tubes” are formed by aperture slots in a stacked assembly of flat, low-conduction baffle plates. Because these apertures collimate both incident x-rays and thermal radiation, such an assembly has been termed a “thermal precollimator.”This paper describes precollimator design principles and design, analysis and testing of a precollimator for the Advanced X-ray Astrophysics Facility (AXAF). NASTRAN and NEVADA were used for thermal modeling, and a full-sized prototype was thermal balance tested to validate the analytical model and the thermal design principles. The paper discusses test methods, analysis versus test correlations, and analysis/test discrepancies. Finally, precollimator effectiveness and applicability of the precollimator thermal design principles to future missions are discussed.