This paper describes a potential new explanation for Alzheimer’s disease (AD), referred to here as the lipid-invasion model. It proposes that AD is primarily caused by the influx of lipids following the breakdown of the blood brain barrier (BBB). The model argues that a principal role of the BBB is to protect the brain from external lipid access. When the BBB is damaged, it allows a mass influx of (mainly albumin-bound) free fatty acids (FFAs) and lipid-rich lipoproteins to the brain, which in turn causes neurodegeneration, amyloidosis, tau tangles and other AD characteristics. The model also argues that, whilst β-amyloid causes neurodegeneration, as is widely argued, its principal role in the disease lies in damaging the BBB. It is the external lipids, entering as a consequence, that are the primary drivers of neurodegeneration in AD., especially FFAs, which induce oxidative stress, stimulate microglia-driven neuroinflammation, and inhibit neurogenesis. Simultaneously, the larger, more lipid-laden lipoproteins, characteristic of the external plasma but not the CNS, cause endosomal-lysosomal abnormalities, amyloidosis and the formation of tau tangles, all characteristic of AD. In most cases (certainly in late-onset, noninherited forms of the disease) amyloidosis and tau tangle formation are consequences of this external lipid invasion, and in many ways more symptomatic of the disease than causative. In support of this, it is argued that the pattern of damage caused by the influx of FFAs into the brain is likely to resemble the neurodegeneration seen in alcohol-related brain damage (ARBD), a disease that shows many similarities to AD, including the areas of the brain it affects. The fact that neurodegeneration is far more pronounced in AD than in ARBD, and characterised by other features, such as amyloidosis and tau tangles, most likely results from the greater heterogeneity of the lipid assault in AD compared with ethanol alone. The lipid-invasion model, described here, arguably provides the first cohesive, multi-factorial explanation of AD that accounts for all currently known major risk factors, and explains all AD-associated pathologies, including those, such as endosomal-lysosomal dysfunction and excessive lipid droplet formation, that are not well-accounted for in other explanation of this disease.