Background and Objectives: High-cost health care users (HCU) represent a minority of patients who consume a large proportion of health care resources. Due to their high burden on the healthcare system and internal heterogeneity, a better understanding of various segments of the HCU population is needed. The general objective was to advance our understanding of incident senior HCUs in the Canadian context so that we can advise health policy makers on potential strategies to prevent seniors transitioning to HCU and to identify priorities for further investigation. Methods: A retrospective population-based matched cohort study was conducted using province-wide linked administrative data. The research employed a spectrum of advanced methods to accomplish the general objective, including the method of recycled predictions, random intercept two-part multi-level models, and stratified logistic regression. Results: Total costs attributable to incident senior HCU status accounted for almost one-tenth of the provincial healthcare budget, with prolonged hospitalizations making a major contribution. Unplanned first (index) hospitalizations (IHs) in the incident year were considerably more common among HCUs, with ten conditions accounting for one third of their total costs. A lower risk of IH among HCUs was associated with residence in long-term care (LTC), attachment to a primary care provider, and recent consultation by a geriatrician. Although there was little variation in costs incurred by Ontario seniors for healthcare services they receive, access to the healthcare services varied greatly. The traditional drivers of costs and mortality (e.g., age, sex, health status) played little role in driving the observed variation in HCUs’ outcomes. Conclusions: By answering research questions, this thesis advances our knowledge of the HCU population in Canada. Further exploration of the nature and quality of care that may be associated with HCU conversion and investigation of the regional variation in accessing specific healthcare services is warranted. / Thesis / Doctor of Philosophy (PhD) / A small group of patients that use the most of healthcare resources are called high-cost users (HCU). HCUs are often seniors. Policy makers need a better understanding of new senior HCUs to be able to prevent seniors from becoming HCU. This study used administrative data and advanced statistical methods. We found that almost one-tenth of the 2013 provincial healthcare budget was spent on new senior HCUs, mainly because of lengthy unplanned hospitalizations. Patients who lived in long-term care, had a primary care provider, or recently visited a geriatrician were less likely to have an unplanned hospitalization. Overall, healthcare costs were distributed equally to Ontario seniors, but access to healthcare services varied greatly. This variation could not be explained by differences in age, sex, or health status. This thesis advances our knowledge of HCUs in Canada. Additional research is needed into care associated with becoming HCU and provincial variation in accessing healthcare.