This document is concerned with reporting the state of the art of measuring impedance in distributed parameter coaxial waveguide systems, propagating a TEM wave. Coaxial lines can be used from direct current to above 40 gigahertz, but are widely utilized beginning with the audio frequencies up through 18 gigahertz. Considered are available accuracies from 50 megahertz to 16 gigahertz, general principles of measurement techniques, basic precautions, and references to pertinent literature. It is one of a series of such reports prepared under a program being carried out by the IEEE. The data in this Report have been assembled by a special task group working in the field of radio-frequency impedance measurements. The data have been organized into tables according to levels or echelons of accuracy ranging from that of the national standard to that of the instruments used by the consumer.<>