Surveillance & Society 1(1): 86-101 Technical Review. Hair Testing: just how accurate Abstract Extensive forensic examination of the hair of 209 "ecstasy" (MDMA) users demonstrated virtually no correlation between self-reported tablet use, and traces of MDMA in the hair of users. Why should this be so? Three answers are possible, and all true. First, self-report is fallible; second, tablet strength varies enormously; and third, forensic analysis is of unknown accuracy. The first two are well known. Forensic analysis, however, typically presents itself as impeccably precise. The review demonstrates that not only is this claim spectacularly untrue, but also that validation of forensic analysis (and, thus, indirectly, self-report) lies in the very blind intra - and inter- laboratory comparisons that are never undertaken.