<p>Police are often becoming primary responders to people experiencing mental illness (PMI), frequently managing crises that fall outside traditional law enforcement roles. Police-PMI encounters are complex, resource-intensive, and often associated with adverse outcomes. Yet the evidence base on how such interactions occur and what influences their outcomes lacks a comprehensive understanding. To address this gap, a systematic literature review was conducted to synthesise the contemporary research on police interactions with PMI within Commonwealth countries over the past decade. Literature searches were conducted across eight databases using predefined inclusion and exclusion criteria. This process resulted in a final sample of 20 peer-reviewed empirical studies. Data were extracted on interaction dynamics, characteristics and experiences of the key parties involved, theoretical perspectives used, methodological approaches employed, and the effectiveness of different response models. The reviewed studies indicate that police-PMI interactions were most commonly crisis-driven, frequently involved individuals with prior police contact, and often resulted in arrest with the use of force or involuntary transport. Across the included literature, police officers commonly reported limited mental health training and unclear operational guidance, while PMI frequently described fear, distrust, and misinterpretation of crisis-related behaviours. Findings related to specialist response models such as co-response and Crises Intervention Teams (CIT) were mixed, with some studies reporting reductions in use of force but limited evidence of reduced arrest or transport rates. Overall, the review identified persistent systemic gaps remain, strained frontline responses, and a limited evidence base, indicating the need for improved training, stronger cross-sector collaboration, and broader, more inclusive research to inform safe and more effective police-PMI interactions.</p>