Cannabis is the illegal substance most commonly used in all countries of the European Union. The increased use of cannabis during the past decade has increased the profile of the drug. Used mainly by young adults, but also by many schoolchildren, cannabis is a drug consumed by individuals during their formative years, at a time when they may be more vulnerable to the long-term harmful effects of drug use. Comments in the media and elsewhere of a large increase in the potency of cannabis have raised concerns that the drug now available is much stronger than that available in the past. To establish a scientific basis on which to advise policymakers and practitioners in the drugs field, the EMCDDA commissioned an investigation into cannabis potency in Europe. The results of this study are presented here.