THIS CHAPTER DESCRIBES MEASUREMENT TECHNIQUES and instru mentation for the measurement of moisture in the air (relative humidity), moisture in building materials, water vapor transmission, water leakage, and building airflow. The chapter does not discuss the extensive literature regarding temperature measurement (an important component in understanding the dynamics of vapor formation and transport in buildings). To do so would only duplicate information contained elsewhere and which is easily obtainable by the practicing engineer. For instance, Ref 1 contains an excellent overview of the various electronic temperature measurement systems and provides detailed references for further study.