Cognitive function in humans declines in essentially all domains starting around age 50-60 and neurodegeneration and dementia seem to be inevitable in all but a few who survive to very old age. Mice with a fraction of the human lifespan show similar cognitive deterioration indicating that specific biological processes rather than time alone are responsible for brain aging. While age-related cognitive dysfunction and dementia in humans are clearly distinct entities and affect different brain regions the aging brain shows the telltale molecular and cellular changes that characterize most neurodegenerative diseases including synaptic loss, dysfunctional autophagy, increased inflammation, and protein aggregation. Remarkably, the aging brain remains plastic and exercise or dietary changes can increase cognitive function in humans and animals, with animal brains showing a reversal of some of the aforementioned biological changes associated with aging. Using heterochronic parabiosis we showed recently that blood-borne factors present in the systemic milieu can inhibit or promote adult neurogenesis in an age-dependent fashion in mice. Accordingly, exposing an old mouse to a young systemic environment or to plasma from young mice increased neurogenesis, synaptic plasticity, and improved contextual fear conditioning and spatial learning and memory. Over the past three years we discovered that factors in blood can actively change the number of new neurons that are being generated in the brain and that local cells in areas were neurons are generated respond to cues from the blood. We have started to identify some of these factors and hope they will allow us to regulate the activity of neural stem cells in the brain and hopefully improve cognition in diseases such as Alzheimer’s.