This investigation is designed to study how the brain functions when it is performing explicit memory tasks, and furthermore how the brain systems supporting performance on these tests change as a function of aging. Implicit memory tasks may be distinguished from explicit memory tests in that explicit memory characterizes the directed or intentional recollection of previously learned information or events. In contrast, implicit (or nonconscious) memory is expressed in the facilitation of performance on some task or judgment on a stimulus owing to a prior presentation of that stimulus, without any necessary conscious awareness of the prior presentation. The investigators propose to use functional Magnetic resonance Imaging (fMRI) to measure brain activity during the performance of carefully designed explicit memory tasks. Comparison of brain activity during the different test conditions will provide information about the brain areas that are involved in mediating specific aspects of performance. Both older and younger individuals will be tested in order to understand age related changes in the brain activity. In order to examine the interactions between participants' differential levels of amyloid burden and their performance on cognitive tasks and fMRI neural activation, the investigators also propose PET imaging of older participants.