Alzheimer's disease is characterized histopathologically by deposition of insoluble forms of the peptide Aβ and the protein tau in brain. Aβ is the principal component of amyloid plaques and tau of neurofibrillary tangles. Familial cases of AD are associated with causal mutations in the gene encoding the amyloid precursor protein, APP, from which the amyloidogenic Aβ peptide is derived, and this supports a role for Aβ in disease. Aβ can promote tau pathology and at the same time its toxicity is also tau-dependent. Aβ can adopt different conformations including soluble oligomers and insoluble fibrillar species present in plaques. We discuss which of these conformations exert toxicity, highlight molecular pathways involved and discuss what has been learned by applying functional genomics.