Publisher Summary This chapter focuses on the occurrence of radon in the atmosphere. The radium-222 distributed in the soil and rocks of the earth's crust produces radon-222, its daughter product, at a constant rate. Once in the atmosphere, the radon isotopes continue to decay and the concentration decreases with height above ground. Radon concentrations in the air just above the soil surface are negligible with respect to concentrations in the soil gas. On the basis of a wide variation in radon flux density and the vertical mixing processes, it can be perceived that radon concentrations in the air near ground level may differ depending upon the location. The patterns of daily and annual changes in outdoor radon-222 concentration have been observed in a general way for many years. Measurements reflecting the mean radon concentrations near ground level at different times of year have been made. Any analytic treatment of the time and space distribution of radon and its daughters must take into account both the decay characteristics of radon-222 and the meteorological character of the environment.