Radio rays are introduced as curves playing a vital role in a short wavelength solution of the equations governing radio propagation in the ionosphere. Taking advantage of the variational equations satisfied by the rays, expressions are found for the variations of the phase path and a number of related quantities due to changes in the ionospheric medium or ray endpoints. The results are applied to the study of group propagation, Doppler shifts, Faraday fading, direction finding, power flow and ray reciprocity.