A brief historical review is given of the investigations on the absorption of radio waves made in the United Kingdom since 1935 together with a detailed discussion of the factors which must be considered when making accurate measurements of this phenomenon. An outline of the theory of ionospheric absorption and its application to practical absorption measurements is included. The effects of double refraction, polarization, spatial attenuation, dispersion, ionospheric inhomogeneities and partial reflection, which modify the apparent attenuation of radio waves reflected in the ionosphere, are discussed. The methods used to minimize errors due to these factors are presented in detail. The experimental techniques used in the routine absorption measurements are described fully. The detailed results of the measurements of the absorption of radio waves in the ionosphere, conducted in South-East England during the years 1935 to 1948, are being published separately.