Radon (222Rn) is a natural noble gas formed by the radioactive decay of the element radium and it is found everywhere in the Earth’s crust. Radon is an invisible gas which is tasteless, odourless and mobile. Since it is barely affected by chemical processes, it can be often used for potential earthquake prediction. Due to these properties, radon is difficult to detect and specific devices are used for its detection and concentration. Radon concentrations in soil and groundwater are higher along active faults and radon concentration abnormalities can be observed several weeks or even months before an earthquake. A potential earthquake prediction is based on the observation of precursor phenomena, but it is difficult to relate an earthquake to any of them. During the measurement of evaporation anomalies of helium, radon, hydrogen and carbon dioxide in some areas of the world that are prone to earthquakes, of all gases radon is the easiest to detect and it is considered the most desirable potential earthquake precursor. Radon anomalies, i.e. increased radon concentrations, were observed in the following countries before an earthquake: Japan, Italy, Taiwan, Slovenia and India. Radon anomalies before an earthquake indicate that radon measurement can be considered a potential earthquake precursor, but the exact association of radon anomalies with seismic activity has not been established yet.