Explicit memory characterizes the directed or intentional recollection of previously learned information or events. In contrast, implicit (or nonconscious) memory is expressed without any conscious awareness of the prior presentation of the stimulus. Functional magnetic resonance imaging (fMRI) will measure brain activity during the performance of carefully designed, explicit memory tasks. Comparison of brain activity during different test conditions will provide information about the brain areas that are involved in specific aspects of performance. Both older and younger individuals will be tested to understand age-related changes in brain activity. To examine the interactions between participants' amyloid burden, performance on cognitive tasks, and fMRI neural activation, the investigators also propose PET imaging of older participants.