The purpose of this paper is to acquaint the reader with some of the basic principles of detailed modelling as applied to combustion systems. Detailed modelling is also known as numerical simulation. It can be used to describe the chemical and physical evolution of a complex reactive flow system by solving numerically the governing time-dependent conservation equations for mass, momentum and energy. Solving these equations requires input data such as the species present, the chemical reactions that can occur, transport coefficients for viscosity, thermal conductivity, molecular diffusion, and thermal diffusion, the equation of state for the various materials present, and a set of boundary, source and initial conditions. Given this information, the equations contain in principle all the information we might want from the largest macroscopic space scales down to the point where the fluid approximation itself breaks down. Flame, detonation, turbulence phenomena, and all multidimensional effects are included in the solutions of these equations. An important goal of detailed modelling is to develop a computational model with a well-understood range of validity. This model can then be used in a predictive role to evaluate the feasibility and validity of new concepts. It can also be used to interpret experimental measurements, to extend our knowledge to new parameter regimes, and perhaps as an engineering design tool. Throughout these various applications, the model may serve as an excellent way to test our understanding of the interactions of the individual physical processes which control the behavior of a reactive flow system.