In the paper the we discuss how the thermal behavior of the Advanced X-ray Astrophysics Facility (AXAF) optical system has been modeled and tested, and how these efforts have influenced the design of the telescope, especially as it relates the imaging performance. This includes the passive/active system covering the space-facing aperture known as the thermal “precollimator”, the mechanical support system that allows the large optical elements to survive the rigors of test in one-G and launch yet minimally affecting on-orbit optical performance, and the active thermal control design of the telescope. Methodologies for the frequently difficult task of transferring results from thermal analysis software to mechanical finite-element analyzers to model thermal deformations are discussed. The complexity of these distortions of the surfaces of the mirror elements required the use of optical raytrace models to assess imaging performance of the telescope. A software tool was developed to allow the transfer of results from the several sources of mirror distortion data to an internally-developed raytrace package to assess performance. Several thermal cases and the resulting telescope performance predictions are shown. Examples of the use of these results in the design cycle to make critical decisions about hardware or the use of ultra-low expansivity materials are discussed.