The final decades of the twentieth century were accompanied by an upsurge in the number of persons fleeing persecution and regional wars. To stem the flow of asylum seekers, several countries in the west introduced policies of deterrence, including detention. Although many countries detain asylum seekers, Australia has been unique in establishing a policy of mandatory, indefinite detention. The impact of prolonged detention on the mental health of asylum seekers drew commentary from mental health professionals soon after the policy was introduced, but administrators and politicians disputed the assertion that detention was a factor in causing or exacerbating mental disorder. This overview examines the impact of mandatory, indefinite detention on the mental health of asylum seekers by drawing on evidence gathered during Commissions of Inquiry, from observations of health and mental health professionals who have worked in detention centres, and from the small body of systematic research undertaken among immigration detainees. The data from all sources converge in demonstrating that prolonged detention has adverse mental health and psychosocial impacts on adults, families and children. Recent studies suggest that the mental health effects may be prolonged, extending well beyond the point of release into the community. The Australian experience offers general lessons to health professionals worldwide about the importance of remaining vigilant in protecting the rights of vulnerable groups, and more specifically, to ensure that the traumas that cause mental suffering in refugees are not compounded as a consequence of immigration policy decisions in recipient countries. Documentation and research can be vital in achieving policy change in these settings.