The study was based on unstructured interviews with 30 inmates in a young offenders institution. Qualitative research with the inmates highlighted the importance of understanding drug use in prison as a continuum of behavior, where inmates' drug use inside was related to their drug use before custody. The inmates made choices and decisions around their drug use inside, considering the compatibility of drugs with the prison environment and their need to seek the "right high." Availability of drugs was crucial. The article discusses routes of drug supply into prison via visits and the informal prison economy, where distribution of drugs was facilitated by close inmate friendship networks. The article emphasizes the importance of knowing about inmates' drug use outside prison in order to evaluate their risk of drug use inside. Equally important is a good local understanding of how an institution potentially influences the pattern of drug use and drug supply, through the regime, searches, and the frequency of mandatory drug tests. Finally, understanding the informal prison economy and the operation of inmate friendship networks offers insight into the distribution of drugs throughout the prison. Notes, references