In Chapter 2, various aspects of waves and transmission lines were summarised. In connection with transmission lines, it was noted that the propagation of TE and TM modes requires certain minimum separation between conductors. It was also noted that waveguides are the key components for guiding electromagnetic energy at microwave range as these can support TE and TM modes. The propagation of these modes becomes a necessity as the TEM mode is not suitable owing to increased attenuation at higher frequencies. Many of the features of waveguides resemble those of transmission lines and the concept of reflection, transmission, current flow in conductor skin, properties of quarter-and half-wave sections, effects of discontinuities and so on hold equally good for both. This is because of the fact that there is an interrelation between transmission line and waveguide. The same is being explored in the subsequent section. Besides the interrelation, this chapter also deals with the mathematical theory of waveguide. In waveguides, the energy is carried out by both electric and magnetic fields. The existence and behaviour of these fields cause energy to travel in accordance with the Poynting theorem. At the end, physical interpretation of various terms, the relative merits and limitations of waveguides are included.