Background: Cognitive reserve (CR) is the concept that was proposed to account for the incongruity in the associations between levels of neuropathology and cognitive function. It is difficult to directly assess CR and so it is frequently indicated by those activities thought to increase it – educational level, occupational complexity, and/or engagement in cognitively-stimulating activities. The evidence regarding the associations and inter-relationships between CR, mood, and cognitive function is conflicting. Previous research has focused on individual proxy measures of CR in investigating these associations; this thesis introduces a novel element by considering CR in terms of multiple proxy measures from across the lifespan in attempting to clarify these complex associations. Method: A meta-analysis synthesised the association between CR and domains of cognitive function in healthy older people. To explore whether CR is associated with self-reported experience of depressive thoughts and symptoms a survey of older people was conducted. To assess whether CR moderated the association between mood and cognitive function, a systematic review of existing evidence and a second survey of older people were conducted. These relationships were then further examined in a large cohort study, representative of the English population aged over 65. Results: Higher levels of the three most commonly used proxy measures of CR, individually and in combination, are all associated with better cognitive function and with decreased subclinical and clinical levels of mood disorders and associated thoughts. CR was found to moderate the associations between mood and cognition across the systematic review and both empirical studies; higher levels of depression and anxiety have greater negative associations with cognitive function in those with lower than higher levels of CR. Conclusions: It is important to continue to build on CR throughout the lifespan in order to maintain cognitive and psychological function and to help mitigate against the negative effect that depression and anxiety can have on cognition.