The finite-difference time-domain (FDTD) algorithm samples the electric and magnetic fields at discrete points both in time and space. The choice of the period of sampling (Δt in time, Δx, Δy, and Δz in space) must comply with certain restrictions to guarantee the stability of the solution. Furthermore, the choice of these parameters determines the accuracy of the solution. This section focuses on the stability analysis. First, the stability concept is illustrated using a simple partial differential equation (PDE) in space and time domain. Next, the Courant-Friedrichs-Lewy (CFL) condition [3] for the FDTD method is discussed, accompanied by a one-dimensional FDTD example.