The FDTD method has gained tremendous popularity in the past decade as a tool for solving Maxwell's equations. It is based on simple formulations that do not require complex asymptotic or Green's functions. Although it solves the problem in time, it can provide frequency-domain responses over a wide band using the Fourier transform. It can easily handle composite geometries consisting of different types of materials including dielectric, magnetic, frequency-dependent, nonlinear, and anisotropic materials. The FDTD technique is easy to implement using parallel computation algorithms. These features of the FDTD method have made it the most attractive technique of CEM for many microwave devices and antenna applications.