The major physical and chemical processes involved in the transport and absorption of O3or NO2In the lower respiratory tract are discussed. This includes the development of respiratory tract models, flow patterns, and transport in tube networks, the mucous, surfactant, and tissue layers, and chemical reactions and transport of O3or NO2within these layers. Descriptions of the individual processes are simplified and Integrated to illustrate the formulation of dosimetry models. Data from a dosimetry model, formulated from the concepts discussed, are used to illustrate the types of information obtained by modeling.