The population of older adults in long-term care (LTC) is expected to increase considerably in the near future. An understanding of the prevalence of psychiatric disorders in LTC will help in planning mental health services for this population. This study reviews the prevalence of common psychiatric disorders in LTC populations. We searched electronic databases for studies on the prevalence of major psychiatric disorders in LTC using medical subject headings and key words. We only included studies using validated measures for diagnosing psychiatric disorders or psychiatric symptoms. Our review focused on the following psychiatric disorders: dementia, behavioral and psychological symptoms of dementia (BPSD), major depression, depressive symptoms, bipolar disorder, anxiety disorders, schizophrenia, and alcohol use disorders. We also determined the prevalence of psychiatric disorders in the U.S. LTC population using data from the 2004 National Nursing Home Survey (NNHS). A total of 74 studies examining the prevalence of psychiatric disorders and psychological symptoms in LTC populations were identified including 30 studies on the prevalence of dementia, 9 studies on behavioral symptoms in dementia, and 26 studies on depression. Most studies involved few LTC facilities and were conducted in developed countries. Dementia had a median prevalence (58%) in studies while the prevalence of BPSD was 78% among individuals with dementia. The median prevalence of major depressive disorder was 10% while the median prevalence of depressive symptoms was 29% among LTC residents. There were few studies on other psychiatric disorders. Results from the 2004 NNHS were consistent with those in the published literature. Dementia, depression and anxiety disorders are the most common psychiatric disorders among older adults in LTC. Many psychiatric disorders appear to be more prevalent in LTC settings when compared to those observed in community-dwelling older adults. Policy-makers and clinicians should be aware of the common psychiatric disorders in LTC and further research into effective prevention and treatments are required for this growing population.