Globally, mental ill health is a growing concern and it's estimated that approximately 1 in 4 individuals will suffer from poor mental health at some stage in their lifetime. Accurate measurement of the prevalence of common mental disorders does not exist, but is necessary for the planning of services, policy development, evaluation, and assessment of needs. Estimates on the magnitude of mental ill health in the population are currently based primarily on survey data. The original contribution to knowledge that this PhD study offers is the introduction of a novel method of measuring population wide mental health in Northern Ireland by utilising antidepressant and anxiolytic drug uptake as proxy indicators of common mood disorder. The thesis generated a dataset containing accurate demographic and prescribing data utilising existing databases, including the national prescribing database, from within the Business Services Organisation. This allowed for population wide analysis of the prevalence of antidepressant and anxiolytic drug uptake. Second ly, this dataset was linked to census returns information from the Northern Ireland Longitudinal Study (NILS) . Both the prescribing data and the NILS contain an individual's unique health and care number allowing for one to one linkage of data sets and the exploration of the individual and area level factors associated with common mood disorder. Most of the variation in antidepressant and anxiolytic drug uptake was explained content not context, suggesting it is who you are not where you live that matters most in relation to common mood disorder. jlastly the prescribing data were linked to administrative data relating to ca re homes for older people to examine the longitudinally the uptake of psychotropic drugs in an older and vulnerable population. This thesis highlights the potential of utilising prescribing data as a proxy indicator of mental ill and as a tool for monitoring prescribing trends in the population.