Hair can be collected under close supervision without embarrassment and is not subject to evasive maneuvers (false negatives) such as temporary abstention, excessive fluid intake, and substitution or adulteration of specimens. Hair analysis has a wide window of detection ranging from months to years and provides information concerning the severity and pattern of an individual's drug use. Hair analysis is also not subject to evidential false positives, such as those caused by poppy seed ingestion, spiking of drinks or food, and mix-up or contamination of specimens. In part, these problems can be avoided because hair analysis can always be repeated with a newly collected specimen. The results of animal experiments and of various clinical, forensic, and criminal justice applications are described. 2 tables, 17 figures, and 45 references. (Author abstract)