Radon is the second leading cause of the lung cancer after cigarette smoking. The book reports measurement of seasonal and spatial variation of indoor radon and development of a passive dosimeter for thoron''s progeny. The book includes procedure of the samples preparation for the measurement of radioactivity and radon exhalation rate in the laboratory using active and passive detectors. The book also reports the results of radon measurement performed in four seasons. Seasonal correction factor, weighted average indoor radon levels excess lung cancer risk and the dose delivered to the inhabitants have also been included in this book. This book also reports the results of radon exhalation from the soil, sand and brick. Measurement of the natural radioactivity in the soil and other building materials, like sand, brick, marble and cement samples has been reported. Experiments using electrets, performance factor for ? contamination chambers using electrets as radon progeny dosimeter(both high sensitivity and low sensitivity electrets). Development of a new passive thorn''s progeny dosimeter using the principle of surface deposition has also been discussed in the book.