The prevalence of mental disorders among both youths and adults has been growing, e.g., the number of people admitted to mental hospitals and expenditures on mental health related medical expenses have doubled over the past 10 years. Health informatics, the health applications of information technology, communication technology, and computer science generally, has long been accepted as a way to improve health services. In this paper, we review current practices and developments in mental health informatics. We broadly categorize mental health informatics into four categories: (1) telemental health (e.g., telepsychiatry), (2) automated diagnosis and assessment of mental health, (3) online mental health support (e.g., mental health social networks), and (4) mental health information management systems (e.g., electronic patient record). In particular, we review research on automated mental health assessment systems, highlighting the potential power of each in solving current mental health care problems.