The purpose of this study is to build upon the information obtained in the original Alzheimer's Disease Neuroimaging Initiative (ADNI1) and ADNI-GO (Grand Opportunity; a study funded through an NIH grant under the American Recovery and Reinvestment Act), to examine how brain imaging technology can be used with other tests to measure the progression of mild cognitive impairment (MCI) and early Alzheimer's disease (AD). ADNI2 seeks to inform the neuroscience of AD. This information will aid in the early detection of AD, and in measuring the effectiveness of treatments in future clinical trials.