Memapsin 2 (β-secretase) is the membrane-anchored aspartic protease that initiates the cleavage of β-amyloid precursor protein (APP) leading to the production of amyloid-β (Aβ), a major factor in the pathogenesis of Alzheimer’s disease (AD). Since memapsin 2 is a major target for the development of inhibitor drugs for AD, it has been intensively studied during the past five years. Here we discuss the structural features of the catalytic/specificity apparatus, transmembrane domain, cytosolic domain and the implications of these features in the physiological and pathological roles of this protease.