The major methods and techniques for measuring radon and radon progeny in indoor environments are reviewed with regard to the measurement principle, field applicability, degree of convenience, cost effectiveness, sensitivity and accuracy. It is pointed out that instruments for the measurement of radon and its progeny are based mostly on the detection of alpha particles and to some extent on the detection of beta and gamma emissions. The instruments and methods used depend on whether the technique measures radon or radon progeny, the type of radiation being detected, and the duration of the measurement, of which there are three types: grab or instantaneous, integrating, and continuous. With the recent increased demand for indoor radon and radon progeny measurements, instrument development has focused on the design of inexpensive mailable devices for short-term exposures used in screening studies, as well as on complex and sophisticated instruments for both short-term and long-term measurements used in research programs. >