Safe disposal of thermally spent geothermalbrines that contain environmentally hazardous constituentsis commonly obtained by reinjection. The reinjectionprocess also serves to maintain reservoir pressure, enhancethermal recovery, and eliminate possible compactionalsubsidence. To avoid premature thermal breakthrough ofreinjected fluids, tracer tests are employed for detectionand evaluation of preferential path networks. In this papersome promising tracers that have not received much attentionin geothermal reservoir studies are discussed, and acomprehensive tabulation of field sites of artificial tracerutilization is presented. Chemical and transport processesresponsible for tracer retention by the formation of reservoirsolids, as well as available tracer detection techniques,are emphasized.