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Alzheimer's disease (AD) is a neurodegenerative disorder that results in cognitive and functional impairment. Current pharmacological treatments have limited effect on correcting cognitive deficits. However, there is a growing amount of literature to suggest that lifestyle factors, such as physical activity, may have a positive effect on cognitive function for people with AD. Through a series of four articles I have addressed methodological short-comings in the existing literature, and determined, through collection and analysis of data in a longitudinal cohort study, the impact of lifestyle factors on cognitive performance in AD. Article I systematically reviews previous physical activity intervention trials and their effects on cognition in an AD population. Physical activity interventions were found to have a moderately positive effect on global cognition. However, the review highlights the apparent heterogeneity between intervention trials as well as the lack of domain specific cognitive outcome measures. Article II focuses on the importance of sensitive measures of cognition in an AD population. Comparing people with AD and age-matched control volunteers, measures of prospective memory were shown to decline with age in the AD volunteers. Significantly, the cost of carrying a PM intention, a measure of working memory, did not exhibit an age related decline and did not differ compared to cognitively healthy controls. Article III explores whether habitual physical activity, is significantly associated with cognitive outcome on a composite measures of executive function. Habitual physical activity significantly accounted for variance (8%) on executive function even after controlling for covariates. Article IV investigates the contribution of habitual physical activity to executive function change in AD over a year. Habitual physical activity was found to be associated with executive function change. These articles contribute in the understanding of the association between habitual physical activity and cognitive function in an AD population.