Mathematical modelling can be a powerful tool in the design and optimalization of glazing. By calculation, the specifications of a glazing design and the optimal design parameters can be predicted without building costly prototypes first. Furthermore, properties which are difficult to measure, like for instance solar and visible light properties for oblique or diffuse irradiation, can be determined accurately by calculation. At the TNO Institute of Applied Physics, a glazing model for simulation and optimalization is being developed, which allows to determine according to international standards, glazing specifications, ranging from optical properties (solar energy, visible light, color appearance) to thermal properties (U-value, thermal shock resistance, fire control properties). The model is connected to a data base containing optical constants and other properties of glasses, coatings and other materials, used in glazing. The paper discusses the Glazing Model which consists of an Optical Transfer Model simulating the optical behavior, and a Thermal Transfer Model which simulates the thermal behavior. The paper gives an overall description of the Glazing Model. A detailed description is given of the matrix formulation which forms the basis for the optical transfer model. An example of a spectrally selective double glazing illustrates how the model works. Present and future developments are also discussed.