This study presents what we believe to be the first formal evaluation of the Taliban crackdown against opium poppy cultivation in Afghanistan. Afghanistan was the main source of the world's illicit heroin supply for most of the 1990s. From late 2000 and the year that followed, the Taliban enforced a ban on poppy farming via threats, forced eradication, and public punishment of transgressors. The result was a 99% reduction in the area of opium poppy farming in Taliban-controlled areas. The evaluation uses multiple comparison areas: the non-Taliban area of Afghanistan, neighbouring countries, the non-contiguous comparison area of Myanmar (Burma), and, the rest of the world. Alternative possible causes of the reduction such as drought, migration or changes in global opium markets are reviewed and excluded. It is concluded that the reduction in Afghan poppy cultivation was due to the enforcement action by the Taliban. Globally, the net result of the intervention produced an estimated 35% reduction in poppy cultivation and a 65% reduction in the potential illicit heroin supply from harvests in 2001. Though Afghan poppy growing returned to previous levels after the fall of the Taliban government, this may have been the most effective drug control action of modern times.