The accuracy of drug use estimates based on self-report data can be monitored by chemical drug testing of biological specimens such as urine, saliva, sweat, and hair. Each biological specimen is unique and offers a somewhat different pattern of information regarding drug use over time. Also, each specimen has strengths and weaknesses regarding the type of information obtained from drug testing. The performance characteristics of the assay methodology may also be important. Urine drug tests represent the most mature technology in the chemical testing of biological specimens. They are of two general types: screening assays, which are commercially based, inexpensive, and simple to perform; and confirmation assays, which are more expensive and more labor intensive, but produce results of higher sensitivity and specificity. The article includes comparisons of urine, saliva, sweat, and hair in the areas of drug detection time, major advantages, major disadvantages, and primary use. Tables, figures, references