This book reviews all the major research accomplishments and summarizes the different applications of radon. It serves as a solid reference book for researchers who are interested in the U-series radionuclides and noble gases as tracers and chronometers. Radon has been widely utilized as a powerful tracer to quantify a number of processes that include gas exchange rates between air and water, submarine groundwater discharge in coastal waters, water exchange between rivers and lakes, ocean circulation, hydrocarbon and uranium exploration. It is also used as an atmospheric tracer for the identification and quantification of air masses and as a tool for earthquake prediction, etc. A significant portion of the book presents state-of-the knowledge on indoor-radon-related health issues. Applications of the decay-series of Rn-222 are presented in a chapter. It serves as a reference and a state-of-the-art resource for researchers who want to learn the different applications of radon in Earth systems.