CO2-rich springs occur worldwide along major zones of seismicity. They are mostly in young orogenic belts, but some are in areas of rifting continental platoforms. Analyses of 13C content indicate that much of the CO2 is derived from the mantle and that other important sources are the metamorphism of marine carbonate-bearing sedimentary rocks and the degradation of organic material. The presence of calc-silicate minerals, such as pumpellyite in metagraywacke, is evidence of former conversion of carbonate-bearing rocks into calc-silicate minerals and release of CO2. The CO2 pressure in fractured rocks of a fault zone reduces the effective normal stress and, if it is sufficiently great, allows the fault to slip. If the pressure were maintained at a sufficently high level, the fault behavior might be characterized by frequent small earthquakes and aseismic creep such as occur along active segments of the San Andreas system. The presence of CO2-rich springs may indicate a potentially hazardous seismic region. Monitoring of CO2 discharges could be useful in earthquake prediction.