The police and the policymakers in the field of organized crime generally assume that criminal groups are very flexible in their responses to law enforcement intervention. They use this flexibility to explain why some forms of organized crime are very difficult to curb. Criminal groups may also develop more effective protection methods over time. Another explanation may be that others are quick to replace apprehended members of criminal groups, or even an entire criminal group that has been dismantled by law enforcement officials. This article addresses these questions empirically by examining the behaviour of criminal groups producing and trafficking ecstasy in the Netherlands between 1997 and 2006. It concludes that such groups were able to adapt to the methods and tactics used by the Dutch police to some extent. The main explanation for the continued production of ecstasy, however, is that criminal groups are replaced by new ones. (Published Abstract)