Neuronal response properties in the cerebral cortex are not static. They can change during development, after deprivation, and following learning. Likewise, even in the adult brain, the structure of neurons is not fixed but can be altered by modifications of sensory input and experience. We study such functional and structural plasticity in the visual cortex, using imaging approaches that allow following the fate of individual neurons over extended periods of time. In my lecture, I will show how such experiments help shed light on the cellular mechanisms underlying specific types of plasticity. I will also present recent data aiming at exploring the potential of transplanted neurons to integrate into existing circuits and take over the function of lost neurons.