Hair testing is used by the FBI only when other information indicates drug use. The procedure has distinct advantages over other forms of toxicological sampling and analysis. The rapid metabolism and elimination of most drugs and their metabolites makes it difficult to analyze and quantify them in body fluids two days or more after use. Cocaine and heroin use can be detected in hair samples collected months after the drugs are consumed. Also significant is the noninvasive nature of hair sampling compared with the collection of blood or urine. Hair tests and urinalysis should be considered complementary. Results of hair testing for drugs of abuse have been found to be consistent with autopsy toxicology reports, and the analysis of hair washes from autopsy samples suggests external contamination of hair with drugs is not widespread. The article includes examples of how testing hair for drugs can be used in a forensic environment. Figures, references