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This thesis explored social and psychosocial factors from different phases in the lifespan and their relation to the occurrence of dementia and Alzheimer s Disease (AD) in late-life. Early life circumstances, such as education, midlife factors such as occupation, and an active and socially integrated late-life were investigated in relation to dementia and AD risk. All five studies are based on the Kungsholmen Project, a longitudinal population-based study of aging and dementia. The initial population consisted of all registered inhabitants who were 75 years and older and living in the Kungsholmen Parish in 1987. The major findings from the five research papers included in this thesis are summarized below. Study I. We found that frequent (daily-weekly) engagement in mental, social, or productive activities was inversely related to dementia incidence. Relative risks (RR) and 95% confidence intervals (CI) were 0.54 (95% CI: 0.34-0.87), 0.58 (95% CI: 0.37-0.91), and 0.58 (95% CI: 0.38-0.91), respectively. Similar results were found when these three factors were analyzed together in the same model. Study II. Less-educated subjects had a RR of 3.4 (95% CI: 2.0-6.0), and subjects with lower socioeconomic status (SES) had a RR of 1.6 (95% CI: 1.0-2.5) of developing AD. Low SES at 20 years of age, even when high at 40 or 60 years, was associated with increased risk. When both education and SES were introduced into the same model, only education remained significantly associated with AD. Study III. A mental, social, and physical component score was estimated for each leisure activity. RR of dementia for subjects with higher mental, physical, and social component score sums were 0.71 (95% CI: 0.49-1.03), 0.61 (95% CI: 0.42-0.87), and 0.68 (95% CI: 0.47-0.99), respectively. The most beneficial effect was present for subjects with high scores in all or in two of the components. Study IV. We found that inactivity, depressive symptoms, but not social network, were independently as