Adverse reactions to implants have provided a framework to evaluate the suitability of polymers, metals, ceramics and composites for applications in tissue or organ repair or replace ment. The lessons learned emphasize the need to many the implant to the structural, functional and temporal requirements oi the host. Implant failure and adverse host reactions have effects that may be local, regional or systsmic. Loss of structural integrity follows corrosion or stress leading to fracture, fragmentation or leaching. Local mobilization of host reactions may produce inflammation, sepsis. coagulopathy. fibrosis and lyric effects. Rarely, neoplasia supervenes. Remote effects include toxicity, altered immune status, allergy or the systemic effects of implant failure or fragmentation, coagulopathy or sepsis. The most serious litigious consequences have implicated putative autoimmune phenomena still mired in controversy. Finally implants should anticipate possible interference with the use of diagnostic or therapeutic modalities.