Amyloid precursor protein (APP), a transmembrane glycoprotein, is the source of amyloidogenic β-amyloid peptide (βAP), the major constituent of the peptides deposited in senile plaques. Normallycleaved forms of APP exert potent neuroprotective action. Accumulating data suggest that an altered expression and/or post-translational processing of APP is responsible for the neuropathological changes observed in Alzheimer’s disease and related disorders. This review discusses the metabolism and secretion of APP, the biological activities of the various APP forms, and the physiological, pathophysiological and experimental circumstances that affect the APP metabolism.