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A review of the current status of national impedance and reflection standards at RF and microwave frequencies is presented. In the first part, various proposals are examined for realizing a given characteristic impedance, or a calculable value of reflection coefficient, both in coaxial lines and in hollow rectangular waveguides. The second part describes measurement techniques and systems used in national standards laboratories to obtain the highest precision and accuracy. Included are techniques using admittance bridges, reflectometers, slotted lines, fixed probes, and network analyzer systems, in particular six-port devices.