Radon and other terrestrial gases in groundwater and soil air have been studied in recent years in seismically active areas, especially in China, Japan, the former Soviet Union, and the United States, in search of changes that may be useful for earthquake prediction. Concentrations of radon, helim, hydrogen, mercury vapor, carbon dioxide, and several other gases have been found to be high along active faults. Faults may thus be paths of least resistance along which terrestrial gases generated or stored in the Earth can escape into the atmosphere. Gas-concentration changes with durations of hours to months have been observed before and after earthquakes at relatively few favorably situated sites, and their patterns have not always been repeated. Some of these sites are located as far as several hundred kilometers away from earthquake epicenters. These “sensitive” sites are generally situated along active faults, whether the seismogenic ones or not, especially at their intersections or bends, or at other structurally weak zones, possibly because of concentration of tectonic strain and abundance of pore fluids at such places (see review papers by King, 1986, 1990). Due to this selectivity and for two other important reasons, studies of terrestrial gases for earthquake prediction have not been viewed favorably by most geophysicists, especially those in the United States.