<h3>Introduction</h3> Arising in the late 1970s as an alternative to nursing homes, Assisted Living Facilities (ALFs) are residential communities for older and disabled adults needing assistance with activities of daily living but in a less restrictive, less institutional/medical setting. Offering a variety of communal home-like living arrangements as well as providing a broad range of health-related services, ALFs are highly heterogeneous. As a whole, they seek to preserve resident independence and autonomy, social wellbeing, and flexibility of service offerings to accommodate evolving resident needs. As of 2016, approximately 811,500 individuals resided in ALFs in the US, across 28,900 facilities with 996,100 licensed beds. The number of licensed beds continues to increase, a trend driven in part by consumer preference; a large national survey indicated that 78% of older adults would prefer to reside in Assisted Living versus 12% in a nursing home in the event they needed more care outside their home. While nursing homes are tightly regulated on the federal level, ALFs are regulated by states and vary widely in their approaches to standards, procedures, and staffing. Regardless of setting, community prevalence of mental illness, defined as "mental, behavioral, or emotional disorders," is 14.1% in adults over 50. In addition, about 11% of people aged 65 and older in the United States have dementia. It is becoming increasingly important to monitor the prevalence of mental illness, including dementia, in ALFs, especially as the older adult population and ALF occupancy increase. The aim of this review is to describe the prevalence of mental illness, including dementia, in Assisted Living residents, to detail existing models of mental health care for Assisted Living residents, and to discuss implications for clinical outcomes, health care utilization, and Assisted Living occupancy. For the purposes of this paper, dementia will be included under the rubric of "mental illness" due to its prevalence among Assisted Living residents, and the overlap of care delivery considerations with non-cognitive mental disorders. <h3>Methods</h3> This narrative review followed the best-practice recommendations outlined by Ferrari (2015). PubMed and Google Scholar were searched in June 2021 for the following key words: assisted living facility, psychiatric illness, limitations, hospitalization, discharge, and behavioral health integration, in addition to synonymous words related to three categories of interest including, prevalence of mental illness in ALFs, behavioral health care models in ALFs, and clinical outcomes of ALF residents. Articles were limited to the English language and to the United States or Canada. Emphasis was placed on recent systematic reviews and randomized controlled trials. All articles were initially screened according to title and abstract and were retained if they contained information on prevalence of mental illness in ALFs, behavioral health care models in ALFs, or clinical outcomes of ALF residents. Reference lists were hand-searched for additional research articles. <h3>Results</h3> Many articles reported various rates of mental illness and cognitive disorders in ALFs, however, the prevalence remains high and continues to increase as the older adult population increases. Across articles, prevalence of non-cognitive psychiatric disorders in ALFs ranged from 26% to 33% depending on methodology and study population. Prevalence of cognitive disorders in ALFs range widely depending on the type of facility: some ALFs admit exclusively persons with dementia, whereas other ALF facilities specialize in residents with fewer functional impairments. Studies have demonstrated that both depression and dementia in ALF residents are more prevalent when compared with community-dwelling controls. Behavioral health problems and high health care needs of ALF residents are common reasons for ALF discharge. Integrated models, which bring together specialty behavioral health care providers with health and supportive service providers, have been shown to reduce behavioral health symptoms and contribute to improved health of residents, improve care, and decrease associate care costs including staff turnover. <h3>Conclusions</h3> This narrative review addresses the importance of behavioral health needs in ALF residents. Articles identified limitations in ALF infrastructure and research, specifically relating to data collection and varying definitions of terms, which limits the generalizability of the results. Greater attention to the study, design, and implementation of behavioral health care for ALF residents may reduce the prevalence of distressing symptoms in those with psychiatric and cognitive disorders, and in turn decrease the need for unnecessary Emergency Department visits, hospitalizations, and premature facility discharge. Future research should consider measuring the effectiveness of mental health care in ALFs since there is limited research currently available. <h3>This research was funded by</h3> Unfunded.