Cathepsin D is the major intracellular aspartic proteinase usually localized in lysosomes. Cathepsin D takes part in intracellular protein catabolism and was implicated in hormone and antigen processing as well as in several pathophysiologic processes including neoplasia and neurodegenerative changes. Cathepsin D plays an important role in the activation as well as the processing of other lysosomal enzymes. The physiological role of cathepsin D is probably broader than just general degradation of intracellular proteins. Cathepsin D appear to be functional in a wide variety of tissues during their remodeling or regression and in apoptotic cell death. The posttranslational modifications of cathepsin D involve several steps of proteolytic processing of its primary structure and sugar moieties. The underlying molecular processes which determine both physiological and patho-physiological functions are still being intensively studied and new information on cathepsin D structure-function relatioship is being published.