**Abstract**

All metal implants in human bodies corrode, which results in metal ion release. This is not necessarily a problem and represents for most patients no hazard. However, both local and systemic effects are possible, including hypersensitivity. To avoid this, coatings on standard implants (mono- or multi-layer) and surface modifications have been developed and are in use. This chapter explains the background of metal ion release, biological reactions, coating technologies, biotribological and biomechanical properties, as well as the clinical results of modern knee arthroplasty implant coatings. There is no general concern about metal ion release from CoCrMo standard implants for most patients. If patients present with a confirmed metal allergy, a multilayer-coated or oxidized zirconium implant is currently the best option for these patients.

**Keywords:** metal ion release, hypersensitivity, coating, multilayer, biological effects
