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Background: The proportion of Canadians aged 65 years of age and older is rapidly growing. While major depression has consistently been identified in the literature as a major health concern, little research exists on geriatric depression in Canada. Current research on risk factors for depression in seniors has been generated almost exclusively through cross-sectional studies. A small number of longitudinal studies have been conducted, though their applicability is limited by short study periods, strictly defined study populations, and focus on only one or two potential risk factors. No longitudinal studies on seniors’ depression have been conducted in Canada at the population level despite the availability of data to do so (the National Population Health Survey, or NPHS). Using the NPHS, this thesis explored the relationship between social support and major depression in Canadian community-dwelling seniors. Methods: I assessed types and levels of social support using the Medical Outcomes Study Social Support Survey. Major depressive episode (MDE) in the past 12-months was assessed using the Composite International Diagnostic Interview – Short Form for Major Depression (CIDI-SFMD) and Anatomic Therapeutic Classification (ATC) Drug codes for anti-depressant usage. Demographic characteristics and health characteristics of respondents and non-respondents were examined and compared. The 12-month prevalence and 2- and 8-year incidence proportions of MDE were estimated. Multivariate logistic regression modeling was used to examine the association between social support and the 8-year risk of MDE Results: In participants aged 65 years of age and older, the 12-month prevalence of MDE was 6.50% (95% CI 5.28%-7.63%). The 2-year and 8-year incidence of MDE was estimated to be 4.54% (95% CI 3.38%-5.60%), and 13.09% (95% CI 11.27%-14.90%), respectively. Incidence was higher among women, those with a chronic condition, those with a restriction to activity, a pain problem or a mobility problem. In multivariate modeling low positive social interaction (OR 1.59, 95% CI 1.12-2.25, p=0.009) and low emotional social support (OR 1.53, 95% CI 1.09-2.14, p=0.013) were significantly associated with the risk of MDE. The association between tangible social support and MDE was modified by income (OR 0.2.70, p=0.019), with those of low income and low tangible support at higher risk of MDE (OR 2.66, 95% CI 1.02-6.89, p=0.044). The relationship between both tangible support (OR 0.43, p=0.025) and affection support (OR 0.35, p=0.008) with MDE was modified by the presence of a pain problem. In both cases, social support was not related to MDE if a pain problem was present. Among those without a pain problem, low tangible social support (OR 2.63, 95% CI 1.03-6.75, p=0.044) and low affection social support (OR 2.09, 95% CI 1.33-3.28, p=0.001) was associated with higher risk of MDE. Conclusion: Social support is an important risk factor for MDE in seniors, even after adjustment for a number of health and demographic variables. The relationship between some types of social support (tangible and affection social support) and MDE may be modified by the presence of a pain problem. It is therefore important that chronic pain issues be adequately controlled in seniors, as these may counter the positive effects of high levels of social support. Living arrangement was not found associated with depression in our study, suggesting that seniors aging in the community versus a long-term care facility are not at increased risk of depression, provided they have high levels of social support. Efforts such as community programs may therefore be important in helping ensure high levels of positive social interaction and social support in community-dwelling seniors.