In order to model open region problems, an absorbing boundary condition (ABC) is often used to truncate the computational domain since the tangential components of the electric field along the outer boundary of the computational domain cannot be updated using the basic Yee algorithm. The quest for an ABC that produces negligible reflections has been, and continues to be, an active area of FDTD research. Most of the popular ABC's can be grouped into those that are derived from differential equations or those that employ a material absorber. Differential-based ABC's are generally obtained by factoring the wave equation and by allowing a solution which permits only outgoing waves. Material-based ABC's, on the other hand, are constructed so that fields are dampened as they propagate into an absorbing medium. Other techniques sometimes used are exact formulations and superabsorption. ABC's tailored for specific applications have also been developed and used with the FDTD method.