The biogenesis of the amyloid-beta peptide (Abeta) is a central issue in Alzheimer's disease (AD) research. Abeta is produced by beta- and gamma-secretases from the amyloid-beta protein precursor (AbetaPP). These proteases are targets for the development of therapeutic compounds to downregulate Abeta production. gamma-secretase has received more attention 1) because it generates the C-terminus of Abeta, which is important in the pathogenesis of AD because the longer Abeta species are more amyloidogenic, and 2) because it cleaves AbetaPP within its transmembrane domain. In the understanding the mechanism of gamma-secretase cleavage, three major cleavage sites have been identified, namely, gamma-cleavage site at Abeta(40/42), zeta-cleavage site at Abeta(46), and epsilon-cleavage site at Abeta(49). Moreover, the novel finding that some of the known gamma-secretase inhibitors inhibit the formation of secreted Abeta(40) and Abeta(42), but cause an intracellular accumulation of long Abeta(46), provided information extremely important for the development of strategies aimed at the design of gamma-secretase inhibitors to prevent and treat AD. In addition, it has been established that the C-terminus of Abeta is generated by a series of sequential cleavages: first, epsilon-cleavage, followed by zeta-cleavage and finally by gamma-cleavage, commencing from the membrane boundary to the middle of the AbetaPP membrane domain.