The paper discusses the use of diagnostic measurements to enhance the selection and effectiveness of radon mitigation for detached dwellings. The development of appropriate and cost effective radon mitigation for a specific house is ultimately tied to the degree to which the authors understand the source of the indoor radon problem, how house characteristics affect radon entry rates, and how candidate mitigation systems influence radon entry processes. Diagnostic measurements and procedures allow for qualitative and quantitative assessments of water, materials, and soil as sources of radon and the assessment of soil-gas-borne radon flow potentials related to indoor/outdoor differential pressure driving forces and soil-air permeability. The discussed diagnostics are being developed and tested as part of an intensive State and Federal government study of radon entry and mitigation processes in 14 Piedmont New Jersey houses.