Mental disorders are characterized by disturbances of thought, perception, affect and behavior, which occur as a result of brain damage. Recognizing and treating these conditions is necessary not only for psychiatrists but for all physicians. Disorder of mental function is one of the most common associated conditions in intensive care unit (ICU) patients. However, disturbances of mental function often remain unrecognized. In ICU patients, different types of mental function disorders may develop. They range from sleep disorders, severe depression, anxiety, posttraumatic stress disorder (PTSD) to cognitive disorders including delirium. The causes of mental dysfunction in ICU patients can be divided into environmental and medical. Cognitive disorders are related to mental processes such as learning ability, memory, perception and problem solving. Cognitive disorders are usually not prominent in the early postoperative period and in many cases are discovered after hospital discharge because of difficulties in performing everyday activities at home or at work. The etiology of postoperative cognitive impairment is unclear. Older age, previous presence of cognitive dysfunction, severity of disease, and polypharmacy with more than four drugs are some of the risk factors identified. Delirium is a multifactorial disorder. It is an acute confusional state characterized by alteration of consciousness with reduced ability to focus, sustain, or shift attention. It is considered as the most common form of mental distress in ICU patients. Nearly 30% of all hospitalized patients pass through deliriant phase during their hospital stay. Delirium can last for several days to several weeks. Almost always it ends with complete withdrawal of psychopathological symptoms. Sometimes it can evolve into a chronic brain syndrome (dementia). The causes are often multifactorial and require a number of measures to ease the symptoms. Delirious patient is at risk of complications of immobility and confusion, leading to a high prevalence of irreversible functional decline. An interdisciplinary approach to delirium should also include family or other caregivers. In the diagnosis of delirium, several tests are used to complement clinical assessment. Among the most commonly used are the Confusion Assessment Method (CAM-ICU) and Clinical Dementia Rating (CDR) test. Depression is a common disorder among patients treated at ICU and occurs due to the impact of the disease on the body and the quality of life, independence, employment and other aspects of life. Depression can interfere with the speed of recovery, affects the postoperative quality of life, and in a certain number of patients may lead to suicidal thoughts and intentions. Phobias and generalized anxiety are the most common anxiety disorders. Generalized anxiety disorder is characterized by strong, excessive anxiety and worry about everyday life events. PTSD is delayed and/or protracted response to a stressful event or situation, extremely threatening or catastrophic nature, which is outside the common experience of people and would hit or traumatize almost all people. Treatment of delirium and other disorders is causal. The causes of psychosis are often multifactorial and require a number of measures to ease symptoms. The primary objective of prevention is appropriate therapy and correction of potential imbalances possibly underlying disturbances, stabilization of vital functions as well as early return to daily activities. Doctors and other medical staff must be aware of the importance and consequences of behavioral and emotional disorders in critically ill patients. Additional research is needed to discover the ways to prevent and/ or reduce the frequency and severity of the consequences and treatment of cognitive and emotional disorders.