Radiative heat transfer process in combustion systems has received relatively little attention to date. Recently, it starts to generate increasing interest given the current trends of engine designs for both internal combustion engines and aeronautical engines. Meanwhile, the need to properly model radiative heat transfer in fire-related scenarios is another driver of the interest. Radiation inside combustion systems is a complex process involving the interactions between spectral gases, soot, droplets, turbulence and the enclosure geometry. Gasses and soot/wall have distinct emission and absorption characteristics, and particles such as spray droplets or water mists have strong scattering effects that might alter the distribution of the heat flux. The turbulent fluctuations in temperature and composition add further complexity to the problem by affecting the production and destruction of soot in a significant manner. The complex geometries encountered in conventional engine or enclosure fires create difficulties in measuring and modeling the heat transfer processes, which hinders the understanding of the physical processes. In this seminar, first-principle-based high-fidelity models are introduced to study the radiative heat transfer process under engine-relevant conditions. Details of the high-fidelity models will be explained, and comparison with popular reduced-order models will be made. The physics will be discussed by means of case studies of recent simulations.