Abstract : After a generel description of the various regions of the terrestrial atmospbere, an analysis of physical conditions leading to the knowledge of the heterosphere is given. Molecular dissociation and diffusion in the lower thermosphere are introduced to determine the concentration of various constituents (N2, O2, O, N, He, H). Ultraviolet heating and conduction are considered as essential processes of production and loss of heat in the thermosphere. Finally, the ionospheric diffusion equilibrium is studied after considering the system of ionic reactions occuring in the ionosphere.