Experiencing treatment on a modern intensive care unit (ICU) is a potentially traumatic event. People who experience traumatic events have an increased risk of depression, anxiety disorders and post-traumatic stress disorder (PTSD). Extended follow-up has confirmed that many patients suffer physical and psychological consequences of the ICU treatment up to 12 months after hospital discharge. PTSD in particular has become increasingly relevant in both the immediate and longer-term follow-up care of these patients. The extent to which the consequences of critical illness and the treatments received in the ICU contribute to the development of PTSD is poorly understood and more rigorous studies are needed. Understanding the factors associated with a poor psychological recovery after critical illness is essential to generate models of causality and prognosis, and to guide the delivery of effective, timely interventions.