Due to the presence of high-temperatures within a combustion chamber, radiation is an essential heat transfer mechanism. Therefore, this chapter investigates the interaction of thermal radiation with gas and solid fuel particles and utilises these findings to develop a novel, computationally efficient model for coupled radiation/computational fluid dynamics (CFD) simulations. The gas radiation model is briefly introduced, while the development of the particle radiation model is explained in detail. In the development of this radiation model, the absorption and scattering coefficients, as well as the scattering phase function, are considered on a single-particle level with subsequent transfer to particle clouds. For this purpose, Maxwell’s equations for a single particle are solved analytically and numerically depending on the problem’s complexity. The final model considers shape and structure influences on the radiative properties depending on the burnout and the type of solid fuel. Furthermore, the influence of each single particle position in a particle group is considered without resolving the particles. The result is a very detailed and efficient particle model.