This study investigated the Mental Health and Social exclusion in People experiencing Homelessness and was completed in two phases; A large cross section study in which six screening assessments and a diagnostic test battery were completed with 529 individuals, followed by an exploratory secondary analysis investigating the association between social exclusion factors and mental health disorders. Analysis included the prevalence of the disorders found within this group, comparisons of the reported disorders with that of the general population and the utility of the screening test administered. A logistic analysis was completed for the 8 mental health disorders to understand the association between these and the 21 factors of social exclusion identified from the work of Percy- Smith and the Multiple Exclusion Project. Prevalence rates within the homeless study were found to be significantly higher than that of the general population with major depression and substance dependency being most prevalent. Psychosis yielded the biggest difference between the two populations. Only the AUDIT and DAST were found to be useful screening tools. Twelve indicators were found to be associated with mental health disorders with odds ratios ranging between 1.20 and 4.43. The study added evidence to the growing awareness of the multiply excluded nature of homelessness. Whether homelessness should be given such prominence in their support is debatable. This research supports the need for services that are multidisciplinary and cater for a broad range of needs. With the current reforms to the health and social care system what seems likely is that without more robust data and assessment, homeless individuals will not receive the services that they so clearly need. Further research is needed into the associated factors of social exclusion and their utility within needs assessments and how services should support those individuals with these complex needs.