Various components contribute to the concept of the vulnerable adult. These typically include learning (developmental) disability and mental illness. Vulnerable adults may have an increased susceptibility to commit crime, be detected by the police and become victims of crime, and therefore coming into contact with the Criminal Justice System (CJS). High rates of psychiatric morbidity may be one such vulnerability factor, especially in adults with developmental disability (DD). Police sergeants use various criteria to identify vulnerable adults. However, reliability and validity of these criteria has not tested. Because vulnerable adults are particularly likely to have difficulty navigating the CJS and may be liable to undue compliance, suggestibility, and misunderstanding, the role of the 'Appropriate Adult' (AA) was created to advocate for identified individuals and support them through the legal process. Two studies, on vulnerable populations in a police station are reported in this thesis. In the first study (main study), a cohort of thirty-four cases and 20 controls were identified at Bethel Street police station in Norwich using the above criteria and prospectively followed up for eighteen-months after initial identification. A similarity between cases and controls in their basic demographics details is reported. There was significant history of unemployment, maternal offending, life events in cases as compared to controls. Relationship difficulties were the most common life event in the month prior to arrestUsing standardised questionnaires including Psychiatric Assessment Schedule for Adults with Developmental Disability (PAS-ADD) high rates of psychiatric morbidity, including anxiety, depression are reported. The key findings are a low sensitivity of the police-screening questionnaire, and high rates of self-claimed mental illness as compared to diagnosable mental illness after using standardised tools. Contrary to other studies, high rates of crimes against the person are reported in both current and past offending among people with DD and mental illness compared to controls. Higher rates of illicit drug abuse and family history of criminality are also reported in cases as compared to controls. Only ten people initially screened completed the study after eighteen months. This reflects the high drop-out rate and difficulties in follow-up of this population. In the second study, one hundred and three adults who were identified as vulnerable by the custody sergeant (CS) at the Bethel street police station were interviewed on a specially designed satisfaction questionnaire. The vulnerable adult population found the Appropriate Adult (AA) to be most beneficial before the police interview. High rates of use of this volunteer scheme are reported with police using more often general vulnerability rather than specific vulnerability factors to screen vulnerable adults