A biostability test program was designed after evaluation of the relevant literature on in vivo aging phenomena in plastics and elastomers. The program comprised macroscopic, microscopic, mechanical, and physicochemical investigations. Five silicone rubbers, one polyester, and one polyamid were tested as to their aging behavior and their suitability for long-term implantation in the human body was assessed. In order to be able to state the applicability of materials used for endotheses, the various aging phenomena had to be divided into three groups, viz. unspecific, relative, and absolute indications of aging or unserviceability. Changes due to aging were found in all types of implanted plastics and elastomers. Thus, the formation of layers on inlay surfaces occurred regularly and the same would apply to changes in mechanical characteristics during the tensile test. Aging processes resulting in total unserviceability were fragmentation and crazing in the polyester and polyamide sheetings. Other aging phenomena which will easily fit into the classification given above are changes in electrical test values, protein, and lipid depositions without stronger absorptive adhesion, crystallizations on the surface of silicone vulcanizates, and chemical changes in the polyester and polyamide sheetings. Following the assessment of the materials used for endotheses, the test methods used have been evaluated with regard to their suitability for the testing of biostability.