The prison population has specific legal needs. While several of these needs are judicial in nature, others are related to personal issues. Whatever the nature of these needs, they should be addressed by the Penitentiary Public Defender's Office, a specialized service of the Public Defender's Office. This article presents the findings obtained from an extensive fieldwork in prisons and penitentiary defense offices in six Chilean regions, with 766 non-participant observations of first interviews in prisons, 616 analysis of folders and of the Computer System of Criminal Defense Management (SIGDP), 19 interviews with Gendarmerie officials and prison defenders, 9 focus groups with prisoners from eight jails, a survey to validate results with 92 responses of defenders and social workers of the penitentiary offices, and a panel of experts, in order to provide an X-ray of the prison defense in Chile. The results show how the exercise of prison defense is limited by a number of factors, but especially, by the overlap of diverse institutional cultures, practices and norms. It is argued that as long as there is no prison law that regulates conflicting institutional expectations and practices, this problem will continue. The absence of law leaves the effective defense of convicts’ rights to the discretion of administrative authorities.