Testing for drugs is important for most clinical and forensic toxicological situations, both for assessing the reality of the exposure/intoxication and for evaluation of the level of drug circulating in the body over a determined period of time. The presence of a drug analyte in a biological specimen can be used to document exposure. In recent years, remarkable advances in sensitive analytical techniques have enabled the analysis of drugs in unconventional biological specimens such as hair. Despite late sampling or even lack of collection of traditional biological fluids, such as blood and/or urine, results for hair testing allow exposure to a drug to be documented. Although there are still controversies on how to interpret the results, particularly concerning children, pure analytical work has reached a plateau, having solved almost all the analytical problems. With respect to hair preparation, only evaluating the pros and cons about the use of pulverized hair versus cut hair and the use of single hair versus a whole strand of hair remains. The aim of this chapter is to discuss issues and controversies, focusing on major recent improvements in new applications, including screening of alcohol addicts, differences between the hair of children and adults, the use of hair to document a single exposure, such as in drug-facilitated crime, the place of hair in doping control in sport, particularly in cases of contamination, and, of course, the oldest challenge in hair testing, i.e., external contamination.