Almost 100 years since the first clinical report of a case of Alzheimer disease (AD), three early-onset and two late-onset AD genes have been identified. While rare mutations in the early-onset genes (amyloid precursor protein, and presenilins 1 and 2) lead to increased generation of specific forms of the amyloid beta protein (A,beta), common polymorphisms in the late-onset genes (apolipoprotein E and alpha 2-macroglobulin) are thought to alter the clearance and degradation of A,beta in brain. Although definite proof for a direct link between altered A beta generation/clearance and neurodegeneration has not yet been attained, mechanism-based approaches for the therapeutic treatment of AD based on lowering levels of the potentially pathogenic A beta are currently underway. The recent discovery of the enzymes (secretases) responsible for generating A beta have paved the way for the development of such drugs and increase the prospects for successful therapeutic intervention to arrest AD neuropathogenesis.