There are several ways to classify electromagnetic waveguides. Depending on their boundaries, they can be classified into waveguides with perfectly reflecting walls or finite impedance walls, as well as open waveguides. In terms of their applications, there are the low attenuation and low delay distortion waveguides for telecommunication, the low crosspolar field waveguides used as feeds for reflector antennas in frequency reuse schemes, as well as the leaky feeders in continuous access communication systems. There are also the natural waveguides such as tunnels and the earth-ionosphere waveguide. This text is an attempt to give a unified, analytical treatment to most of these diverse types of waveguides. Emphasis is given to modal characteristics, modal excitation and mode scattering at longitudinal discontinuities. The book will serve not only as a text for undergraduate courses, but also as a reference book for those involved in research work on electromagnetic waveguides.