Alzheimer's disease (AD) is the most common cause of dementia and will lead to a worldwide public health crisis. Before 2011, the most widely used and best validated diagnostic criteria for AD were the NINCDS-ADRDA (National Institute of Neurological and Communicative Disorders and Stroke - Alzheimer's Disease and Related Disorders Association) criteria published in 1984. Alzheimer's research has greatly evolved over the past decades. Major scientific advances in clinical, genetics, biochemistry, molecular biology, and neuroimaging have suggested that the brain changes that cause AD can be detected before a person becomes demented. The National Institute on Aging and the Alzheimer's Association updated the diagnostic guidelines covering the full spectrum of AD in 2011. The newly proposed criteria reflect the evidence of the time course of pathology and biomarkers in relation to clinical symptoms of AD. The new criteria emphasize that the AD pathophysiological process starts years and perhaps decades before clinical symptoms, and that biomarkers can detect amyloid β deposition and the effects of neuronal injury in the brain. These guidelines describe the 3 stages of AD: the earliest preclinical stage of AD, mild cognitive impairment due to AD, and dementia due to AD. The new guidelines address the use of neuroimaging and biomarkers in the cerebral spinal fluid that may help to determine whether changes in the brain are due to Alzheimer's disease. The most advance of the new criteria is to propose the concept of preclinical AD. Although part of new criteria is intended purely for research purposes, it changes our view about the disease process of AD.