High-temperature gas cooled reactor (HTGR) designs present special computational challenges related to their core physics and thermal-hydraulic characteristics. Neutron streaming and double heterogeneities are examples of the core physics challenges, while porous gas flow in a high-temperature ceramic core with the interaction of conduction, convection and radiation regimes characterize thermal-hydraulic challenges. In an effort to address such potential calculation uncertainties, computational results made by various international institutes are reported and code-to-code and code-to-experiment comparisons are made. The experimental data are mainly related to the initial testing of the Japanese HTTR and the Chinese HTR-10. Both are high-temperature gas cooled test reactors. The work is part of an IAEA Co-ordinated Research Project (CRP) with the participation of ten international institutes.