Polyurethanes have become the most valuable implantable elastomers for uses requiring toughness, durability, biocompatibility and biostability. They are inherently stable in the body environment. However, physical and chemical changes may be effected by conditions of processing, fabrication, use or interactions with other device components. Most prominent modes of polyurethane degradation include mineralization, environmental stress-cracking and oxidation. While the mechanisms of these forms of degradation are not fully understood, an awareness of their causes and effects can lead to procedures that provide all of the long-term functionality required for the sophisticated polyurethane-based devices of today and tomorrow.