Polycrystalline graphite has a unique combination of high-temperature properties that has made it the material of choice for many industrial applications. Several nuclear reactor designs that operate between 500 C and 1,000 C include graphite components. These components must maintain their integrity even at the 1,800 C they could be exposed to during an accident. The operational behavior of these graphites during both proof testing of as-manufactured material and postirradiation examination must be determined by measuring physical, mechanical, and thermal properties. For reasons of expense and practicality the properties are measured in (or near to) ambient conditions. It is essential that the measured properties may be extrapolated reliably to high temperatures. Laboratory testing at elevated temperatures therefore provides data for (1) defining temperature-dependent extrapolation curves,(2) informing conceptual models that help to establish confidence in ambient-temperature test methods, and (3) inputs into numerical simulations of operating conditions. The properties of interest for this paper are selected on the basis of current ASTM standards to include those most relevant to current and future fission reactor operation. The effects of fast neutron irradiation on the high-temperature behavior are presented in general terms, and the conventional understanding of the mechanisms behind both the inert and irradiated behavior are outlined. Areas for further research are then highlighted, the findings of which would support design, qualification, operation, and safety monitoring of graphite-moderated nuclear reactors.