The CHANDRA X-ray Observatory (formerly AXAF), one of NASA's “Great Observatories” was launched aboard the Shuttle in July 1999. CHANDRA comprises a grazing-incidence X-ray telescope of unprecedented focal length, collecting area and angular resolution - better than two orders of magnitude improvement in imaging performance over any previous soft X-ray (0.1-10 keV) mission. Two focal-plane instruments, one with a 150°K passively-cooled detector, provide celestial X-ray images and spectra.Thermal control of CHANDRA includes active systems for the telescope mirror and environment and the optical bench, and largely passive systems for the focal plane instruments. Performance testing of these thermal control systems required 1-1/2 years at increasing levels of integration, culminating in thermal-balance testing of the fully-configured observatory during the summer of 1998. This paper outlines details of thermal design tradeoffs and methods for both the Observatory and the two focal-plane instruments, the thermal verification philosophy of the Chandra program (what to test and at what level), and summarizes the results of the instrument, optical system and observatory testing.