The paper gives results of a 12-month field study on the mechanisms of radon entry and the effectiveness of mitigation techniques in 14 similar houses in New Jersey. Continuous monitoring of a variety of parameters before and after mitigation has provided a very rich data base. Continuous data were obtained for radon concentrations and temperatures in different zones in the house, under the slab, and in the walls, for pressure differences across the basement perimeter, for operation of the central air distribution system, and for basic weather variables. Several-day averages of air infiltration rates, both between individual zones and outdoor air, and between pairs of zones, were obtained throughout the period. The mitigation systems tests were sub-slab pressurization and depressurization with and without sealing, basement sealing, and basement pressurization. An evaluation of the efficacy of mitigation techniques tested in two single-story test houses is presented as well as a discussion of how mitigation affected air infiltration and radon source strengths. The amount of air infiltrating each basement from the soil gas is also discussed.