Many older adults are experiencing unmet needs for assistance with the activities of daily living (ADLs) and instrumental activities of daily living (IADLs). Such unmet needs might threaten their physical and psychosocial well-being. We conducted a systematic review to provide a comprehensive picture of the health consequences of unmet ADL/IADL needs among older adults. Twenty-eight published articles were included for qualitative synthesis. We found that unmet ADL/IADL needs were consistently associated with higher health care utilization (e.g., hospitalization, medical spending) and adverse psychosocial consequences (e.g., anxiety, depression), while the findings of falls and mortality remain inconsistent. More studies are needed to draw firm conclusions and to allow for quantitative synthesis. This review advocates for more coordinated and comprehensive long-term care services for older adults. Future studies should explore how the adverse health outcomes identified in this review can be prevented or improved by adequately meeting older adults' needs for assistance.