This book is about the key elements of the millimeter-wave technology: various millimeter-wave waveguides, waveguide transitions, and devices. The book can serve both as a tutorial presenting the basic theory and the main experimental techniques necessary for the work with millimeter-wave waveguides and millimeter-wave devices, as well as a monograph presenting new developments in this field. Examples show the use of millimeter-wave waveguides in the design of microwave devices and antennas.