T h e most common cause of endodontic failure has been ascribed to incomplete obturation. Filling a root canal is intended to prevent microorganisms and toxins in the canal, as well as those present in the oral cavity, from passing along the root canal and moving out into the periradicular tissues. Matsumiya and Kitamura both demonstrated that bacteria remain in the deep portions of the endodontic system, even after chemical disinfectants. Healing of the periapical tissues occurs when the nutritional supply to the bacteria is stopped by the sealing properties of obturation. Unfortunately, the endodontic system is complex, including the accessory canals, anastamoses, and apical deltas. The inability to completely obliterate the irregularities of the endodontic system with a filling material and seal the apical delta foramen has been reported to account for nearly 60% of root-canal failures. Fluid percolation into the canal arises indirectly from the blood serum, water-soluble proteins, enzymes, and salts from both the apex and oral cavity. Once in the radicular space, those products undergo degradation and diffuse out to the periapical tissues and act as an irritant, which results in periapical inflammation. Bacteria will result in the producuon of toxic products in the root canal with periapical inflammation. In current endodontic treatment, it is said that "it is more important what comes out of the canal than what goes into it." So the emphasis seems to be placed more on cleaning and preparing the endodontic system than on filling it. This does not mean that obturation is less important; rather it means that endodontic success depends on meticulous root-canal preparation. For a complete three-dimensional fill, proper instrumentation of the root canal makes it a lot easier to facilitate. The objective of obturation of the radicular space is to fill the entire endodontic system and all of its complex anatomic pathways completely and densely with a nonirritating hermetic sealing agent(s), which results in a "fluid-tight seal," especially at the apex, but also coronally (to prevent infection from the oral cavity). Success is dependent on the thoroughness of endodontic cavity design, shaping, and cleaning. The thoroughness of debridement and patency of the root-canal system facilitates its successful sealing in the three-dimensional phase. A three-dimensional fill is desired because it: 1) prevents percolation of the periapical exudate into the root-canal space (providing a culture medium for any residual bacteria); 2) prevents microorganisms from entering and reinfecting the root canal; and 3)