The ridge waveguide, which is a rectangular waveguide with one or more metal inserts (ridges), is an important transmission line in microwave engineering, through which many passive components can be achieved. As such it is a well-established and widely used element in commercial electronics and communications devices. This book collects together much of the work of Professor Helszajn, an international authoriy in the field, and will enable the reader to have direct access to this material without need for exhaustive search of research papers. Generously illustrated, it is likely to become the definitive reference source on this topic. The book includes closed-form and finite element calculations of the propagation constant, attenuation and mode spectrum for the ridge waveguide, as well as power-current and power-voltage definitions of impedance. Circular polarisation is also treated. Propagation properties where the waveguide has a dielectric filler are calculated. The treatment is then extended to more complex designs, including quadruple ridge waveguides with and without a gyromagnetic filler. The text includes descriptions of many of the passive devices which can be realised using these waveguides, including isolators, phase shifters and circulators. A treatment of the finline waveguide is included as its geometry is closely related to that of the ridge waveguide, leading to components such as the 3-port finline calculator.