During growth and keratinization certain chemicals, including drugs, are "trapped" in hair. These chemicals can subsequently be released and recovered for analysis. Hair analysis can provide long term information, from months to years, concerning drug exposure in an individual. Both modern and ancient hair samples have proven useful in analyses. This paper will examine scientific issues such as the incorporation of drugs into the hair shaft, external contamination, etc. in addition to a report on analysis of 18 hair samples from Dakhleh Oasis, Egypt