In recent years we have seen the emergence of commercial applications at high frequencies, such as the top part of the microwave band and the millimeter and sub-millimeter bands, and it is expected a big increase in the coming years. This growing demand requires a rapid development of low-cost technology with good performance at these frequencies, where common technologies, such as microstrip and standard waveguides, have some shortcomings. In particular, existing solutions for high-gain planar scanning antennas at these frequencies suffer from the disadvantages of these technologies giving rise to high-cost products not suitable for high volume production. This book, therefore, provides, on one hand, the study of a new proposal to improve existing solutions to date for low-cost high-gain planar scanning antennas at high frequencies, and on the other, the study of new quasi-TEM waveguides that are more appropriate than current technologies for the realization of circuits and components at these frequency bands. These guiding solutions make use of periodic or artificial surfaces in order to confine and channel the fields within these waveguides.