Overcrowding in prisons is a common problem that affects many countries. It is difficult to define this term because there is no single internationally accepted standard. However, this is a situation that must be counteracted, because people's behaviour can be affected to the extent that it leads to self-harm or violent behaviour to others. But prison overcrowding also has other effects on the health and well being of the people living in these conditions and may also adversely affect public health and the prison system. It can increase the prevalence of diseases, particularly infectious and psychiatric disorders. It may also hinder the work of social rehabilitation and lead to inhuman, cruel or degrading treatment. This paper reviews the scant literature about overcrowding in prisons. Increasing awareness about the issue in the international community and prison administrations and above all, assigning specific economic resources, are key elements in preventing this deficit in social welfare and health care.