Alzheimer disease (AD) is a neurodegenerative disease that affects cognition, behavior and function. The etiology of the disease is unknown, however, the Primary Risk Factors for AD are aging, and family history. Neurofibrillary tangles (NFT) and amyloid-bearing neuritic plaques in the limbic and cerebral cortices are the characteristic neuropathologic lesions in brains of patients with AD. The NFT is mainly composed of hyprephosphorylated tau, whereas the major component of the neuritic plaques is the amyloid beta (Aβ) protein. The clinical diagnosis of probable AD is based on history, physical examination, neuropsychological testing, laboratory studies and neuroimaging techniques. However, there is no specific laboratory marker to support the diagnosis of definite AD or monitoring the progression of the disease. Several biochemical markers related to neuropathology have been identified in cerebrospinal fluid (CSF). We describe the studies of CSF or blood levels of amyloid β protein in patients with AD and age-matched nondemented controls. Due to the heterogeneity and complex nature of the disease, it is highly unlikely that that a single marker specific for AD will be found.