The most notable contributions to our understanding of the D and E regions of the ionosphere made during the period from 1967 to 1971 have come through the following developments: the determination of the ionic composition of the ionosphere through the use of mass spectrometers flown on sounding rockets; the measurement of electron density profiles under a wide range of conditions, at various latitudes, hours, and seasons, during eclipses and winter anomalies with probes in combination with radio receivers for propagation measurements flown on sounding rockets; the measurement of the concentration of several important minor species in the mesosphere and lower thermosphere, such as NO, O2(a1 Δg), and Na; the explosive outburst of laboratory measurements of rate constants for reactions between electrons, ions, and molecules of atmospheric interest. These developments have been accompanied by some significant successes and some significant failures in theoretical studies attempting to reconcile the measurements.