The shape memory effect, superelasticity, good wear and damping properties make the nickel-titanium shape memory alloy (NiTi) a fascinating material for many surgical applications. It provides a possibility to make self-locking, -expanding and -compressing implants. As porous NiTi has elastic modulus near the bone tissue, it may also be a appropriate candidate as a bone substitute material. Theoretically, biocompatibility problems may arise because of high nickel content of NiTi. To estimate the in vivo biocompatibility it is essential to do testing and analyzing in different tissues as the environmental conditions (pH, electrolytes, protein composition etc.) inside the human body vary a lot. When certain specific applications are concern the biomechanical questions are also of great interest. The overall inflammatory response and the encapsule membrane thickness around to Nitinol in recent studies have been found to be similar to that of stainless steel (StSt) and Ti-6Al-4V alloy. So far, there has been no reports of tissue necroses, granulomas, or signs of dystrophic soft tissue clacification in vivo. The general immune cell response to Nitinol has been found to remain low also during a long term implantation. The characteristics of muscular tissue response to NiTi are nontoxic, nearly inert. Muscle tissue response to porous NiTi has been found to exhibited a thin, tightly adherent fibrous capsules with fibers penetrating into implant pores. The neural and perineural responses reported are nontoxic and nonirritating after NiTi implantation. Nitinol wire might be a very promising new tendon suture material because of high strength comparing the conventional materials. The biocompatibility of NiTi in tendon tissue is excellent. NiTi has no negative effect on new bone formation and the bone contact to NiTi has been found to be very close, indicating good tissue tolerance, The osteotomy healing with porous NiTi has also found to be good. A typical bone remodeling, characterized by normal osteoclastic and osteoblastic activity has been reported. After long term implantation no difference in nickel concentration between the NiTi and StSt groups has been found in any of the distant organs, The surface corrosion changes of retrieved NiTi implants were minimal. Based on the published studies, NiTi appears to have good potential for clinical use as its biocompatibility also in vivo is good. When NiTi is intended to be used in long-term implants, for example vascular stents, optimal surface treatment must consider.