The application of a very sensitive radar system to the observation of signals incoherently scattered in the ionosphere is briefly described. This technique can give more detailed information about height and time variations of bottom-side electron-density profiles than any other method, besides measuring several further parameters of the plasma. Electron-density profiles of the day-time lower Eregion, and of the whole of the night-time Eregion, allow much more reliable estimates than hitherto to be made of nondeviative h.f. absorption, and of the reflection heights of m.f. and l.f. waves. The morphology of the ‘valley’ in the profile between the E and Fregions can be investigated; also the ability to measure details of profile changes during ionospheric disturbances should allow the accompanying deterioration in h.f. propagation conditions to be understood. Incoherent-scatter measurements give unique opportunities of making independent calibrations of statellite instruments in orbit, and of measuring plasma movements induced by neutral aid winds. An empirical law relating the temperature and density of the electrons has been discovered; this will help in predicting the changes in density which should occur when additonal heat energy is deposited in the ionosphere during disturbances.