Alzheimer's disease (AD) is a neurodegenerative disorder with a complex etiology and pathogenesis. Mutations in presenilin 1 gene (PSEN1), located on chromosome 14, more rarely in amyloid-beta protein precursor (APP) on chromosome 21, and presenilin 2 genes (PSEN2) on chromosome 1, underlie the pathogenesis of most cases of familial early onset of AD (EOAD). The genetics of late-onset AD (LOAD) have been more enigmatic and the only confirmed risk factor for LOAD remains the apolipoprotein E4 allele (ApoE4) on chromosome 19. In this review, we discuss the genetics of AD with a focus on the role of the APP and presenilins.