In this paper we review the theory of acoustic-gravity waves, the interaction of such waves with the ionosphere, the experimental support for the existence of such waves in the upper atmosphere, and the role played by acoustic-gravity waves in thermospheric dynamics. After a thorough discussion on the properties of acoustic-gravity waves in an ideal isothermal atmosphere, the effects produced by horizontal winds, sharp boundary discontinuities, and dissipative processes are discussed. The generation of these waves by stationary or moving sources is then treated. It is shown that the atmospheric response to a stationary impulse source can be described by the emission of three waves: acoustic, buoyancy, and gravity. These discussions are then followed by reviewing propagation effects in a realistic atmosphere for both free waves and guided waves. Recent numerical results are given. When acoustic-gravity waves propagate through the ionosphere, interaction between the wave and the ionosphere will take place. The physical processes involved in such an interaction are examined. The response of the ionosphere to acoustic-gravity waves can be fairly complex, but its understanding is necessary to interpret various experimental data. The existing experimental data on traveling disturbances are then reviewed. The existence of acoustic-gravity waves throughout the atmosphere implies coupling between the lower atmosphere and the upper atmosphere. Transport of both momentum and energy are accompanied by the wave process. The implication of momentum and energy transport on thermospheric dynamics is discussed.