The paper discusses the state of the art of alternative technologies for reducing radon concentrations in the living areas of houses with crawl-space foundations. For the two most promising technologies-sub-membrane depressurization (SMD) and crawl-space depressurization-the effects of individual house design, house operation, system design, system operation, and geological variables on system performance have been evaluated. SMD is consistently the most effective technique. Evaluation of the effects of the various house and mitigation system variables indicates that the major data gaps are: (1) the conditions under which crawl-space depressurization might be preferred over SMD; (2) the optimal method for distributing suction beneath a SMD membrane under different conditions; (3) the optimal degree of SMD membrane sealing required under different conditions; (4) the conditions under which it may be possible to leave a portion of the crawl-space uncovered; and (5) the durability of SMD the other systems.