The explication of neurofibrillary tangles is inextricably tied to the protein tau and the pathology of Alzheimer’s disease. Neurofibrillary tangles, as one of the pathological hallmarks of Alzheimer’s disease, are important to the study of abnormal and clinical psychology. After summarizing the history of Alois Alzheimer’s description of neurofibrillary tangles, this entry discusses their primary constituent, abnormal hyperphosphorylated tau protein. In addition, the role of neurofibrillary tangles in the neuropathological diagnosis of Alzheimer’s disease is briefly discussed.