Morphological, serological and chemical examination of human hair for medical purposes was initiated some decades ago. In the 1960s and 1970s, hair analysis was used to evaluate exposure to toxic heavy metals. At this time, examination of hair for organic substances, especially drugs, was not possible because analytical methods were not sensitive enough. Since the early 1980s, the development of highly sensitive and specific assay methods such as radioimmunoassay or gas chromatography/mass spectrometry has permitted the analysis of organic substances trapped in hair. This, theoretically, offered the possibility of revealing an individual's recent history of drug exposure beginning at sampling day and dating back over a period of weeks or months. The present review aims to summarise the various applications that have been published.