The processes responsible for primary tissue failure of bioprostheses were illustrated. Calcification is the major cause of failure of porcine bioprosthetic valves, usually with superimposed tearing. Calcific deposits may be found grossly or microscopically in most bioprostheses removed from adult humans after four years. The predominate loci are the cuspal commissures and basal attachments, in the porcine spongiosa layer, in cells and in collagen. Pericardial valves also tear frequently, most often near the top of the stent post. The status of understanding of bioprosthetic valve calcification and studies of preventive therapies were summarized. Data concerning retardation of mineralization by use of diphosphonate in controlled-released drug delivering polymers and pretreatment with aluminum salts were presented.