INVESTIGATORS IN FEDERAL AND STATE government, universities, and private industry are making radon measurements using different measurement devices placed in different locations during various ventilation conditions and over different time periods. These public health officials, mitigators, researchers, and building engineers are measuring radon and decay products using a variety of practices, leading to sometimes vastly different results even in the same building. It is important for anyone measuring radon to recognize that different measurement practices lead to measurement results that may not be comparable. The specifications for the factors that can greatly influence the result of a measurement are defined as a protocol. It is the intent of this chapter to assist the reader in evaluating the type of protocol that is appropriate for his or her needs. The chapter discusses the different procedures and guidelines in use by different groups but does not necessarily recommend their use.