Although mental and physical health are likely to share common social causes, most mental-physical comorbidity research has focused on immediate mechanisms between mental and physical illness. This thesis takes a social epidemiological approach to mental-physical comorbidity, where social disadvantage and the disproportionate availability of resources are central. The amplified burden of comorbidity in terms of poor health and functioning may have implications for the relationship between comorbidity and mental health service use (MHSU). Whilst much research examines the impact of comorbidity on physical health services, MHSU is under-researched. Furthermore, comorbidity inequalities may be perpetuated through processes of cumulative disadvantage. For example, barriers to social participation may deplete resources over time, thus leading to worse health outcomes and more adverse social circumstances. This project therefore aims to: 1. Estimate the prevalence of comorbidity, and describe inequalities in mental-physical comorbidity by key socio-demographic and socioeconomic factors 2. Describe and explain the association between comorbidity and mental health service utilisation and quality 3. Describe the trajectories of social functioning by comorbidity Analyses make use of survey data from the South East London Community Health Survey (SELCoH) phases 1 (N=1698) and 2 (N=1052) (73% response among those approached at follow-up). Statistical methods used include weighted cross-sectional and longitudinal regression analyses. The results indicate that comorbidity is associated with distinct socio-economic inequalities (most consistently by household income), increased MHSU over time, and persistent social exclusion. This suggests that comorbidity reflects a process of cumulative disadvantage, which has important implications for comorbidity and health inequality research, and local services and policy makers. Altering the downward spiralling trajectories of health and social disadvantage among those with mental-physical comorbidity may be addressed through integrated care models, while interventions aimed at reducing social inequalities may effectively 3 prevent comorbidity and interrupt its downward spiralling course of disadvantage.