Isotopic measurements can be used for tracing the sources and behavior of environmental contaminants. Perchlorate (ClO4 -) has been detected widely in groundwater, soils, fertilizers, plants, milk, and human urine since 1997, when improved analytical methods for analyzing ClO4 - concentration became available for routine use. Perchlorate ingestion poses a risk to human health because of its interference with thyroidal hormone production. Consequently, methods for isotopic analysis of ClO4 - have been developed and applied to assist evaluation of the origin and migration of this common contaminant. Isotopic data are now available for stable isotopes of oxygen and chlorine, as well as 36Cl isotopic abundances, in ClO4 - samples from a variety of natural and synthetic sources. These isotopic data provide a basis for distinguishing sources of ClO4 - found in the environment, and for understanding the origin of natural ClO4 -. In addition, the isotope effects of microbial ClO4 - reduction have been measured in laboratory and field experiments, providing a tool for assessing ClO4 - attenuation in the environment. Isotopic data have been used successfully in some areas for identifying major sources of ClO4 - contamination in drinking water supplies. Questions about the origin and global biogeochemical cycle of natural ClO4 - remain to be addressed; such work would benefit from the development of methods for preparation and isotopic analysis of ClO4 - in samples with low concentrations and complex matrices.