During the past 10 years, clinicians have used two-piece implants as a one-piece system for immediate loading to accomplish early recovery for function and esthetics. The statistical analysis by the Institute of Clinical Materials demonstrates high survival rates of 92.3 ± 8.3%. Animal experiments of beagle dogs have been carried out to analyze successful conditions of immediate loading according to the dependency of bone formation–resorption upon the biomechanical stress–strain of bone. Histometric investigations clarified that the micromotion of less than 30 &mgr;m at the implant–bone interface did not interfere with the osteogenesis and new bone growth at the implant–bone interface. Bone formation and resorption are discussed from the standpoint of minimum effective strain on living bone.