Cell culture has many uses in the design of implants. It can be used for biocompatibility assessment as well as tissue engineering applications. For biocompatibility, tests can be used to assess or understand cellular mechanisms in response to implants; determine toxicity of implant materials, and determine stimulatory properties of implant materials and included substances. For tissue engineering, the principles are similar. In this case cells, biomaterials, and biological response modifiers are used to replace or stimulate regeneration of biological structures. Studies are needed to determine the optimal environments for each component. This requires either simulating the in vivo environment or actually growing structures in vivo.