In Great Britain, interest in the subject of drugs and drug users in prison has culminated in various policy initiatives. Drawing on semi-structured interviews with key players in the policy process and analysis of documentary materials, the article explores the historical shifts within prison drugs policy, locates those changes within a wider policy context, and assesses the role and influence of policy networks. Since 1980, prison drugs policy can be divided into four main phases of development which have hinged upon complex patterns of conflict, contradiction, and convergence between treatment and punishment. Throughout the phases, particular policy networks have evolved around drug-related issues within prisons, expanding and becoming more complex in their structure and operation over time. The networks' role in relation to setting the policy agenda and responding to it has shifted over the various phases. The article concludes that the networks' power lies in their attempts to contain, balance, and negotiate the key contradiction between treatment and punishment within prison drugs policy. Table, notes, references