Some fifty years ago, the first experiments were conducted that provided unequivocal evidence for the existence of an electrically charged region in the upper atmosphere capable of reflecting radio waves. Appleton and Barnett, in England, and Breit and Tuve, in the United States, conducted expeximents in which the height of the reflecting layer could be reduced. They observed echoes from several heights between 100 and 200 km. Some years later Watson-Watt coined the term ionosphere for this reflecting region. These early experiments led to the development of instruments, known as vertical-incidence sounders or ionosondes, to perform routine sounding of the reflection characteristic of the ionosphere, and a worldwide network of sounders continues in operation. The results these instruments obtained constituted the most important source of information on the properties of the atmosphere above 100-km altitude prior to about 1950, when high-altitude rocket research became practical. In the last 25 years, there has been an explosive growth in our knowledge of the properties of the upper atmosphere raising from in situ measurements made with rockets and satellites. Radio-wave investigations have continued to make very important contributions, however, owing to the development of a new sounding technique known as incoherent scatter. In this method a very-high-power VHF or UHF radar is employed to observe the weak signals backscattered by density fluctuations in the ionosphere resulting from the random thermal motion of the electrons and ions. Unlike the older reflection technique, this permits the variation of the electron concentration to be measured to very great altitudes (≥ 10 000 km); moreover, the temperatures of the electrons and the ions may be determined separately. Over certain altitude ranges the following additional information may be obtained: the concentration, temperature, and horizontal wind velocity of the neutral particles; the intensity and direction of the polarization electric field (established by the neutral wind and/or the interaction of plasma from the sun with the geomagnetic field); the transport of ionization and heat to, or from, the magnetosphere (i.e., the outermost portion of the ionized envelope surrounding the earth); information on the production of photoelectrons by the sun. The paper provides a brief summary of the early reflection experiments, and outlines the principles and practice of the new method. The power of the incoherent scatter technique is illustrated by examples of recent results. In conclusion, current plans to construct large new radars to study the auroral ionosphere are described.