Aeronomy is the study of the physics and chemistry of the upper atmosphere. The upper atmosphere is usually defined as the region of the atmosphere above the tropopause extending upward to the point where electric and magnetic fields dominate the phenomena rather than the atmospheric atoms and molecules. The lower part of this region, from about 10 to 90 kilometers altitude, has become known as the middle atmosphere. An international program called MAP (Middle Atmosphere Program) is now underway to intensively study this region. Three reviews of work in the middle atmosphere appear in this volume covering the composition, dynamics, and electrodynamics. Susan Solomon's paper, “Minor Constituents in the Stratosphere and Mesosphere,” documents the continued growth in knowledge concerning the composition of the middle atmosphere, the mechanisms which maintain this composition, and its possible response to outside influences. Dennis Hartman's review, “Middle Atmosphere Dynamics,” examines the large-scale dynamics and climatology of the middle atmosphere, particularly pointing out the importance of the introduction of transformed Eulerian mean equations for dynamics and transport and the realization of the importance of gravity waves for the momentum budget of the stratosphere. Michael Kelley's review, “Middle Atmosphere Electrodynamics,” discusses a variety of new techniques that have been used to obtain “existing and controversial” results including large (several volt/meter) fair weather electric fields in the mesosphere.