Muscle transfer is one of the breakthrough improvements in reconstructive microsurgery because of its “ability of transferring function.” The muscle flaps are frequently applied in reconstructive surgery by transferring them without separating their vascular and neural connection, however, the usage of innervated free muscle flaps in upper extremity will be referred to in this chapter. Adjacent muscles may not always be useful for replacing the function of injured muscles even if they are undamaged. In other words, a disorder becomes clinically observable when the injury is not compensated by the other muscle compartments. Therefore, innervated free muscle transfers become substantial in case of incapability of local treatment methods such as tendon transfers.