Ten million people are currently incarcerated around the world. In prison, psychiatric disorders are 4 to 10 times more frequent than in the general population. Beyond the many ethical and societal issues raised by these observations, specific structures have been developed in France (SMPR, UHSA). Within these structures, psychiatric practice and clinical assessment must be based on the same recommendations as outside the prison. However, it appears that several specific clinical features are encountered. These features emerge first from a “selection bias”, resulting in the presence of patients with a predominance of “positive” symptoms (particularly for bipolar disorder and schizophrenia), that has been associated with an increased risk of forensic act. The other specific clinical features of incarcerated patients come from the particular characteristics of the prison environment which is a concentrate of stressors. Nonspecific symptoms (anxiety, insomnia, suicidal thoughts, etc.), and specific symptoms related to the characteristics of the prison (detention conditions, links with the prison administration, etc.) will be highlighted. Two examples will be developed: self-harm and delusions of persecution. It appears essential for health professionals who evaluate incarcerated patients or working in prisons, to know the specificities and constraints of life in prison, to incorporate these aspects in a rigorous clinical evaluation.