A general account is given of British work on ionospheric exploration, conducted by vertical-incidence radio sounding, during the period from 1924 to date. The essential feature of such exploration is the determination of the equivalent height of reflection as a function of radiowave frequency, from which relation the electronic structure of the ionosphere may be approximately deduced. From such verticalincidence measurements it is possible to predict the refractive influence of the ionosphere on waves obliquely incident on it, and thus to estimate, for given ionospheric conditions, the maximum radio frequency which is returned to the ground at any given range from a sending station. Moreover, such estimated maximum usable frequencies for oblique-incident transmissions may themselves be based on a prediction of ionospheric structure made some months in advance. Such advance forecasts of ionospheric-layer electron densities and heights must be based on past experience of diurnal and seasonal behaviour and of the general waxing and waning of ionization densities during the sunspot cycle.Since 1935, measurements of ionospheric layer heights and ionization densities have been supplemented by regular determinations of overall ionospheric reflection coefficients at vertical incidence. In this way some progress has been made in estimating the attenuation of radio waves in practical cases of oblique-incidence reflection. It cannot, however, be claimed that the predictions of ionospheric absorption are yet as reliable as the predictions of ionospheric refraction.