Another characteristic of Alzheimer’s disease in the brain is the accumulation of neurofibrillary tangles consisting of a protein called tau. In its normal form, tau has the important role of supporting the structure of microtubules in neurons which helps support neuronal function. In Alzheimer's disease and some other neurodegenerative diseases, tau becomes abnormal and forms protein aggregates. In aging and Alzheimer’s these take the form of neurofibrillary tangles, which appear to be closely related to brain degeneration (atrophy, or shrinkage of the brain), changes in brain metabolism, and cognitive decline. In addition to Alzheimer’s diseae, the abnormal tau is commonly seen in the brains of healthy cognitively normal older people, where it is found in the hippocampus and entorhinal cortex, brain regions closely associated with memory function. A major hypothesis is that, in the presence of brain Aβ deposition, tau spreads out of the memory system and into other parts of the brain, leading to more severe cognitive decline. Tau PET imaging utilizes a radioactive tracer to bind to neurofibrillary tangles and image their distribution in the brain. By combining measurements of tau with amyloid PET and MRI measures of brain function and structure, we can investigate relationships between tau deposition and memory loss in normal aging and dementia.