This report was prepared, at the request of the U.S. Environmental Protection Agency, for the purpose of reviewing, assessing, and summarizing what is currently known about radon transport through and exhalation from building materials. In four chapters, the report (1) considers the routes of entry of radon into buildings, describes the basic models for radon transport through building materials, critically reviews the small number of existing values for the necessary transport coefficients, and summarizes the solutions of both steady-state and time-dependent transport cases; (2) reviews and considers how the microstructural properties and internal characteristics of building materials may affect the transport and exhalation of radon; (3) considers the exhalation process from a more macroscopic, phenomenological viewpoint, and summarizes selected experimental data on radium concentrations in building materials, radon flux and exhalation from soils and building materials, and the effects of meteorological variables on radon exhalation; and (4) reviews and assesses various measurement methodologies that are used for laboratory and in situ studies of radon transport and exhalation. Needs for further research in each area are also recommended.