A description is given of the construction and the method of operation of the rotary attenuator, and it is shown that, since the measurement of attenuation by this instrument is dependent inherently upon the measurement of an angle, it is an absolute device. By the use of scattering coefficients and considering the various discontinuities within the instrument, the errors that can arise in its use are analysed. It is shown that the most serious source of error is due to insufficient maximum attenuation, which can be due to a limitation of the centre vane or to misalignment between the end vanes. All other errors can be measured and made negligible by good microwave measuring practice.Finally, some details are given of the methods that can be used to check the law of the attenuator and practical accuracies, and results are presented for an instrument operating in the 3 cm waveband which has been compared with a precision piston attenuator operating at 60 Mc/s.