Indoor radon reduction measures were tested in 40 existing houses with significantly evaluated radon concentrations in eastern Pennsylvania. In all but one, soil gas was the predominant source of the radon. The houses all had basements, sometimes with an adjoining slab-on-grade or crawl-space wing. Most of the radon mitigation techniques involved some form of active soil ventilation. In addition, three heat recovery ventilators (HRVs) were tested, and two carbon filters were tested for removing radon from well water. The tests showed that significant radon reductions (90 - 99%) can be achieved with properly designed active soil ventilation systems. In basement houses with concrete floor slabs, suction on perimeter drain tiles can be very effective when a reasonably complete loop of drain tiles exist. Sub-slab suction (with individual suction pipes penetrating the sub-slab region) would be the next technique of choice, though it can be important that the suction pipes be carefully located when sub-slab permeability is poor. Ventilation of block wall cavities can give less predictable results. HRVs can provide moderate radon reductions (usually no greater than about 50% for reasonably sized HRVs), although their effectiveness in different parts of a house cannot always be reliably predicted. Carbon filtration can remove significant amounts of radon from water (up to 95-99%), at least over the 9-month period that they were tested in this study. The source of the carbon can be very important.