Advances in the understanding of Alzheimer's disease (AD) pathogenesis provide strong support for a modified version of the amyloid cascade hypothesis, which is now often referred to as the amyloid beta protein (Abeta) cascade hypothesis. The basic tenant of this modified hypothesis is that Abeta aggregates trigger a complex pathological cascade leading to neurodegeneration. Thus, as opposed to the original amyloid hypothesis, whose basic tenant was that amyloid deposits cause AD, the Abeta hypothesis is more inclusive in that it takes into account the possibility that several different Abeta assemblies might contribute to AD pathogenesis and not merely the detectable amyloid deposits within the brain. Significantly, the Abeta hypothesis has provided the rationale for a plethora of therapeutic interventions that target Abeta production, aggregation or clearance. Indeed, AD research is entering an exciting phase in which strategies derived from basic research will be tested in humans. Despite this progress, many aspects of AD pathogenesis, particularly those downstream of Abeta accumulation are not well understood. Herein, we explore several observations that serve to illustrate the more enigmatic aspects of the Abeta hypothesis, and discuss why further basic research may be critical in order to develop therapies designed to halt neurodegeneration and reverse cognitive decline in patients already suffering from AD dementia.