Since late-1984, EPA's AEERL has supported a program to develop and demonstrate radon mitigation techniques for single-family detached dwellings. As part of the program, projects have been started directed at developing and demonstrating the use of diagnostic measurements in all phases of the radon mitigation process. Diagnostic measurements are used to assess: (1) the radon sources' strengths, variability, and locations; and, (2) radon transport to the house and its entry and distribution in the house as influenced by environmental, house characteristics, and occupancy factors. The diagnostic measurements reported include: (1) soil-gas grab sampling; (2) communication (air flow or pressure field extension) tests; (3) whole house infiltration; (4) differential pressure, (5) gamma radiation; and, (6) radon flux. The paper concludes that the above selected diagnostic measurements have been found especially useful in characterizing houses which have indoor radon problems attributable to soil-gas-borne radon which may be amenable to mitigation through the use of subslab ventilation.