Summary In the year 2000, the world’s prison population numbered around 8.7 million; less than two decades on, it stands at well over 11 million. The steady growth in prisoner numbers has given rise to severe prison overcrowding in much of the world, in both developed and less developed countries. Our World Prison Brief data show that prison systems are now overcrowded in well over 60 per cent of the world’s countries. When prison population growth is not matched by increased investment in built capacity, staff and other resources (as it almost never is), living conditions quickly deteriorate, as do standards of sanitation, healthcare, wellbeing and safety. The consequences for health – not only the health of prisoners and staff but also of their families and wider society – are serious. Prison as both site and cause of ill health People who enter prison are already more likely to have poor health than the general population. This reflects the fact that prison populations are disproportionately made up of people who come from the poorest and most marginalised sections of society, where prevalence of mental and physical health problems tends to be greater and health inequality more marked. In addition, in most societies today it is all too easy for people with mental health conditions, or who are dependent on drugs or alcohol, to be propelled into the criminal justice system and custody. Prisons carry heavy burdens of both communicable and non-communicable diseases, burdens they are ill-equipped to bear. Although imprisonment occasionally allows people who otherwise have no access to healthcare to receive some treatment, time spent in custody is far more likely to worsen existing health problems and give rise to new ones. Prisons are also difficult settings in which to provide treatment and healthcare. They are even less capable of meeting the complex health needs of the many prisoners who have specific vulnerabilities: older prisoners, women, young prisoners, people with disabilities, LGBT prisoners. This report contains many descriptions by prisoners and former prisoners of cramped, insanitary conditions and unsafe environments where distress, trauma, self-harm, anxiety and violence are the norm. These accounts attest to the damage that prison so often causes to people’s physical and mental health. They describe living conditions where inmates cannot even count on the basics: drinking water, proper toilets, cleaning materials, food, and sleeping space. Prisoners describe their struggles to access medication and treatment, even for serious conditions like diabetes, tuberculosis and HIV. Mothers with infants in prison describe how their children regularly fell ill with diarrhoea, cholera and skin diseases. The sleeplessness, anxiety, confusion and fear that prisoners experienced in custody continued long after their return to the community. A sentence of imprisonment is a sentence of deprivation of liberty, not of damage to health. States running prison systems that leave people in worse health than when they entered custody cannot expect prisons to achieve rehabilitation or make communities safer. They will simply perpetuate the revolving door of ill health, unemployment, poverty, homelessness – and repeated arrests and prison sentences. Governments that fail in their duty to protect prisoners’ physical and mental health and provide adequate health treatment and decent, humane conditions create unacceptable public health and security risks. This leaves them open to legal challenges for infringing fundamental and constitutional rights. Towards a health-informed approach to penal reform For most countries, reducing prisoner numbers overall is an essential first step to improving conditions of imprisonment and increasing access to healthcare and treatment for those in custody. This requires reform at the criminal justice level and more broadly. (This is the focus of our wider prisons research, though not of this report.) A key part of reducing overall prison populations is to take every available step to reduce the numbers of people with mental health conditions who enter custody. We give examples of interventions capable of diverting people v