Since the earliest descriptions of Alzheimer's disease (AD), many theories have been advanced as to its cause. These include: (1) exacerbation of aging, (2) degeneration of anatomical pathways, including the cholinergic and cortico-cortical pathways, (3) an environmental factor such as exposure to aluminium, head injury, or malnutrition, (4) genetic factors including mutations of amyloid precursor protein (APP) and presenilin (PSEN) genes and allelic variation in apolipoprotein E (Apo E), (5) mitochondrial dysfunction, (6) a compromised blood brain barrier, (7) immune system dysfunction, and (8) infectious agents. This review discusses the evidence for and against each of these theories and concludes that AD is a multifactorial disorder in which genetic and environmental risk factors interact to increase the rate of normal aging ('allostatic load'). The consequent degeneration of neurons and blood vessels results in the formation of abnormally aggregated 'reactive' proteins such as β-amyloid (Aβ) and tau. Gene mutations influence the outcome of age-related neuronal degeneration to cause early onset familial AD (EO-FAD). Where gene mutations are absent and a combination of risk factors present, Aβ and tau only slowly accumulate not overwhelming cellular protection systems until later in life causing late-onset sporadic AD (LO-SAD). Aβ and tau spread through the brain via cell to cell transfer along anatomical pathways, variation in the pathways of spread leading to the disease heterogeneity characteristic of AD.