Isotopic measurements can be used for tracing the sources and behavior of environmental contaminants. Perchlorate (ClO ₄ ⁻ ) has been detected widely in groundwater, soils, fertilizers, plants, milk, and human urine since 1997, when improved analytical methods for analyzing ClO ₄ ⁻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 ClO ₄ ⁻ 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 ClO ₄ ⁻ samples from a variety of natural and synthetic sources. These isotopic data provide a basis for distinguishing sources of ClO ₄ ⁻ found in the environment, and for understanding the origin of natural ClO ₄ ⁻ . In addition, the isotope effects of microbial ClO ₄ ⁻ reduction have been measured in laboratory and field experiments, providing a tool for assessing ClO ₄ ⁻ attenuation in the environment. Isotopic data have been used successfully in some areas for identifying major sources of ClO ₄ ⁻ contamination in drinking water supplies. Questions about the origin and global biogeochemical cycle of natural ClO ₄ ⁻ remain to be addressed; such work would benefit from the development of methods for preparation and isotopic analysis of ClO ₄ ⁻ in samples with low concentrations and complex matrices.