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Advancing age is associated with marked atrophy in prefrontal brain structures coupled with behavioural differences across multiple cognitive domains, and yet evidence from functional neuroimaging frequently indicates overactivation for older relative to young adults. This rather paradoxical result, arising from the unimodal application of different techniques, has led to multiple theories of cognitive ageing, where each of the theories provides a unique interpretation of its technique-related findings. To overcome this we need to develop multimodal approaches where the relations between brain function and behaviour can be mapped at molecular, functional and behavioural levels. In particular, investigating the links between neuronal activity, the neurochemical environment and the effects of ageing on behaviour are essential to progress in understanding age-related deficits in cognition. Localizing neural activity with functional magnetic imaging (fMRI) provides good spatial resolution and is useful to the application of magnetic resonance spectroscopy (MRS), where a priori knowledge of the brain function is needed to investigate the concentration levels of neurotransmitters. One of the neurotransmitters measured by MRS is the main inhibitory neurotransmitter γ-Aminobyturic Acid (GABA), which has been suggested to play an important role in the modulation of cognition, neuronal activity and ageing. Therefore, one of the main aims of the thesis is to correlate GABA levels to neural activity and cognitive performance by combining GABA-edited MRS and BOLD fMRI correlates in response to a specific cognitive task in young and older adults. Furthermore, existing cognitive theories in ageing were extended with predictions about the multimodal data acquired in the thesis.