This article focuses on the evolution of prison library services in the United States and the changes in the roles and purposes of prison libraries over the last two centuries. The development of standards and guidelines for prison libraries under the leadership of the American Library Association and the American Correctional Association is discussed. The characteristics of the offender population are described as well as how prison libraries have responded to the specific needs of this special user group. The challenges of the unique prison environment are highlighted, especially as they relate to the delivery of library services. Examples of successful library services and programs are included, with descriptions of technology projects, resources for prison library staff, collection development policies, law library services, literacy programs, and resources to assist inmates with the transition back to society.