Implanted biomedical devices are of increasing importance in modern medical care. However, surprisingly little is known of the factors that determine biocompatibility of the materials used in these devices. These materials, although generally inert and non-toxic, can mediate a variety of adverse reactions, including inflammation, fibrosis, coagulation, and infection. This brief review focuses on the inflammatory responses (including fibrosis) that commonly occur around implanted biomaterials. Host proteins that spontaneously associate with implant surfaces are important determinants of the acute inflammatory response. In this regard, adsorbed fibrinogen appears particularly pro-inflammatory. Chronic inflammatory processes, in many cases in response to fragments of implanted biomaterials, may cause implant failure. In the case of silicone-filled mammary prostheses, the extravasation of silicone gel has been held responsible for a number of complications, including silicone granuloma, synovitis, connective-tissue disease, and lymphadenopathy. In some instances, material-mediated inflammatory responses may even cause degradation of the material itself (via oxidative products released by implant-associated inflammatory cells). Overall, there is insufficient knowledge of the determinants and mechanisms of host: implant responses. A clear understanding of tissue:biomaterial interactions will be required both to explain the pathogenesis of many implant-mediated complications and to aid in the development of more biocompatible materials for implantable devices.