At higher frequencies where wavelength becomes small with respect to feature size, it is often necessary to consider an electronic signal as an electromagnetic wave and the structure where this signal exists as a waveguide. A variety of different concepts can be used to examine this wave behavior. The most simplistic view is transmission line theory, where propagation is considered in a one-dimensional (1-D) manner and the cross-sectional variation of the guided wave is entirely represented in terms of distributed transmission parameters in an equivalent circuit. This is the starting point for transmission line theory that is commonly used to design microwave circuits. In other guided wave structures, such as enclosed waveguides, it is more appropriate to examine the concepts of wave propagation from the perspective of Maxwell’s equations, the solutions of which will explicitly demonstrate the cross-sectional dependence of the guided wave structure.