Mental hygiene consists of measures to reduce the incidence of mental illness through prevention and early treatment and promote mental health. This unit focuses on basic principles of mental hygiene, indicators of mental health, warning signs of poor mental health, features of mental illness and causes for poor mental health. A nurse has a key role in improving mental health among the population. To fulfill this role effectively a detailed description has been given on mental health across the life span and the nurse’s role. Nurses are in a key position to identify individual, family and group needs, conflicts and stressors. Thus, they play a major role in identifying high risk groups and preventing the occurrence of mental illness in them. To provide adequate preventive services, this unit examines nurses’ role in primary, secondary and tertiary prevention.