Prisons in modern societies are complex bureaucracies that are subject to regulation through a range of external monitoring and internal inspection mechanisms. Internal inspections are closely connected to the bureaucracies that manage individual prison systems. External monitoring is conducted by international, regional, and national organizations. In theory, international monitoring by organs of the United Nations has a worldwide reach, but its impact in a particular country depends on whether the country has ratified the relevant international instrument and agreed to being monitored. Regional monitoring is particularly prominent in Europe. It is less prominent in other regions. The form that national external monitoring adopts varies greatly between countries. It may be done by a specially created inspectorate, human rights committee, or ombudsman. Local monitoring committees for individual prisons, complaints procedures, and court actions may all serve regulatory functions. These diverse bodies are most effective as monitoring organizations when they are independent, apply clear standards, and have political support. Their efficacy may be increased by international support and the acceptance at the national level of the salience of human rights standards for prison conditions. (Published Abstract)