The paper discusses diagnostic approaches that offer improved evaluations of radon-related indoor air quality (IAQ) problems. An informed solution involves knowledge of the building, the building site, and the interaction of radon sources with the living space. The diagnostics can be viewed as having applicability in four phases of the mitigation process: (1) diagnostics which assess the radon problem; (2) premitigation diagnostics, from which a suitable mitigation approach must be chosen; (3) important diagnostics that check the performance of the radon mitigation solution; and (4) diagnostics that determine if the radon problem has been solved (that guideline values have not been exceeded over the different seasonal conditions experienced). A consensus of current knowledge on important radon diagnostic techniques and how they may be best applied are the result of a two-day Radon Diagnostics Workshop sponsored by the U.S. EPA held at Princeton University, April 13-14, 1987. The paper will summarize those findings, placing the various radon diagnostic techniques in perspective.