Human brains and cognitive functioning are in a constant flux of change throughout life. The question is: can you decide to what extent your brain and cognition will change, and how? This has enormous implications - it is a question of by which mechanisms humans can adapt to their changing environments with changing minds. And it is a question of how to handle the frequent cognitive problems experienced by especially elderly adults. Research has yielded astonishingly different perspectives on cognitive and brain changes through life. On the one hand, studies point to brain development and aging being under genetic control. On the other hand, there are associations between intellectual and physical experiences and cognitive function across the lifespan, and recent studies show that brain and cognition are improved by targeted cognitive interventions. However, the time course, stability, generalizability and restrictions to training effects on brain and cognition are largely unknown. The aim of this proposal is to uncover mechanisms governing neurocognitive plasticity - its potential, restrictions and time course - in young and old age, and reconcile the apparent contradiction between genetic control and environmental impact. I will study the effects of memory training with repeated Magnetic Resonance Imaging and cognitive tests in a new experimental time-series cross-over design with 200 young (20-30 yrs of age) and 200 elderly (70-80 yrs of age) adults. Neurocognitive changes in controls not training are compared to those in participants undergoing alternate repeated periods of memory training and rest (A-B-A-B) for one year, with a three-year follow up. This will allow me to identify 1) distinct modulators of plastic changes in terms of age, neural integrity, and genotype, 2) the time course of plastic changes in brain and cognition, their stability across short and long time, and 3) the extent of transfer of memory training effects to other cognitive functions.