[en] Radon is of current interest from several points of view: as a radiation source in water, buildings, and mines; as a disturbing agent in low-activity measurements; as a tracer element for radium in man; etc. For many years, measurements have been made at the Swedish National Institute of Radiation Protection (NIRP) on radon and its occurrence and behavior in nature. Appropriate sampling and measuring techniques are described for radon in water and air, intended for field as well as laboratory measurements. Some observations on radon concentrations in outdoor air, water, and buildings are reported. Special attention is given to the experience of measurements in mines. The observed concentrations are reported and discussed, as well as the health aspects of the exposure to radon and radon-daughters in mines