Electromagnetic waves are all around us, from radio communication to radar. Their propagation is governed by Maxwell equations, which forms a linear system. The solution to Maxwell equations are modes of the system. Maxwell equations are driven by current sources as in antennas. These are linear systems driven by additive control. We can excite a system in free space or in a closed cavity by driving it on its resonant frequency. In this chapter, we develop the analogy between Maxwell equations and linear systems.