Thermal conductivity and thermal diffusivity of refractory oxide ceramic materials with porosities 5-98% vary in a complicated manner with temperature, T, and gas pressure, p. These variations can not be explained on the basis of known classical heat transfer mechanisms in porous materials, such as heat conduction in solid and gas phases, radiation and gas convection within the pores. This paper reviews these and additional mechanisms and processes affecting heat transfer in ceramic materials that have been recently investigated, to explain and rationalize thė p- and T- dependences of oxide ceramics in the ranges 500 K<T<2500 K, 10^-2 Pa<p<2^.10⁷ Pa. Two main groups of mechanisms are considered: 1. Heterogeneous heat and mass transfer processes occurring in pores existing at grain boundaries, and in cracks, in particular, surface segregation and diffusion of impurities on pore surfaces, transport of gases produced from chemical reactions, evaporation, and sublimation. 2. Microstructural changes due to nonuniform thermal expansion of particles and grains. These changes are caused by mismatch between the thermal expansion coefficients of different material phases, and anisotropic thermal expansion of crystals. We discuss the predominant factors, which together with temperature and gas pressure affect thermal conductivity of several commonly used ceramics materials. These factors include porosity, geometrical parameters of pores, cracks and grain boundaries. A physico-mathematical model for calculation of thermal conductivity of composite porous materials is described and used to explain and correlate the extensive experimental data collected for this quantity in wide ranges of gas pressure and material temperatures. The model incorporates all relevant heat transfer mechanisms and physico-chemical processes occurring within ceramics and allows to explain the complicated pressure and temperature dependences of the material thermal conductivity. Several practical applications of the model are also outlined.