A general mathematical model is presented for predicting the concentration and fate of particulate matter in indoor air. Using a multicomponent sectional representation, the model accounts for the effects of ventilation, filteration, deposition onto surfaces, direct emission, and coagulation. Model predictions are compared with the evolution over time of the measured aerosol size distribution following combustion of a cigarette in a single room with a low air-exchange rate. Reasonable agreement is obtained; however, further experiments are required for full validation of additional capabilities of the present model. Important environmental problems to which the model may be applied include analysis of the soiling of surfaces due to deposition of airborne particles and control of human exposure to environmental tobacco smoke. The model may also serve as a foundation for improving the understanding of the risk of human exposure to radon decay products indoors.