A device that constrains or guides the propagation of electromagnetic radiation along a path defined by the physical construction of the guide. Electromagnetic waves may propagate in space, as radio waves, but for many purposes waves need to be guided with minimum loss from the generating point to a point of application. Several guiding systems are of importance, including two-conductor transmission lines, various forms of striplines used in microwave integrated circuits, hollow-pipe waveguides, and dielectric wave­guides. Since transmission lines and striplines are discussed in other articles, the emphasis in this article is on hollow-pipe and dielectric guides. Hollow-pipe guides are used primarily in the microwave region of the spectrum, dielectric guides primarily in the optical region.