Human hair analysis is now recognized for evaluating someone exposure to xenobiotics: drugs of abuse, pharmaceuticals and polluants. This paper describes analytical methods than can be used by biologists. For drugs of abuse after decontamination, hydrolysis of hair, selective extraction and derivatization, determinations are performed by gas chromatography-mass spectrometry (GC/MS). Calibration uses deuterated standards. The cut-off value is 0.5 ng/mg for 6-monoacetylmorphine (heroin) and amphetamines and a benzoylecgonine/cocaine ratio > 0.05 for cocaine. Measurement of metabolites from endogenous metabolism sign the exposure (6-monoacetylmorphine and morphine for heroin, benzoylecgonine for cocaine, delta 9-tetrahydrocannabinol carboxylic by-product for cannabis). For drugs, after selective extraction, determinations are performed by GC/MS or liquid chromatography coupled to a diode array detector. Main applications concern drug monitoring to complement blood determinations or when blood collection is missing, as evidence of hidden, illicit or criminal drug exposure. Finally it is a powerfull tool for clinical diagnosis especially when late biological investigations are performed.