In recent years, disrupting the supply of illicit drugs and injecting equipment inside Her Majesty's (HM) prisons has become an important focus for prison drug policy. This paper presents findings from qualitative research, which invited 24 drug injectors with prison experience to discuss the role and operation of illicit drug and injecting equipment markets inside prison. These data were obtained from in-depth interviews and small group discussions. The analyses of these findings are grouped under three broad themes. First, why and how drugs and injecting equipment enter prison from outside. Second, the maintenance of supplies inside prison. Third, the availability of drugs and injecting equipment with a special focus on the quality of injecting equipment. These findings raise important implications, not least public health concerns with the transmission of infection. Prison policy should take a much stronger lead in reducing the harms caused by the operation of prison drug and injecting equipment markets.