Recent progress in theoretical and experimental work on clear air propagation on line-of-sight paths is reviewed. Descriptions of the physical mechanisms involved, illustrations of the variability in time and in space of the phenomena, and descriptions of the modeling of the propagation are addressed. Absorption by atmospheric gases is dealt with briefly. Refractive index structures are mentioned as background information. Theoretical and experimental studies of the physical modeling of multipath propagation conditions are dealt with in a detailed form. Various examples of multipath parameters modeling measured propagation conditions are presented. Aspects of statistical descriptions of multipath characteristics are also treated. Impacts on radio communications systems are mentioned. Attention is called to the difference between physical models of the propagation behavior and simplified models relating statistical characteristics. Duct theory and duct propagation characteristics are described. Examples of valuable statistical descriptions of radiometeorological data are mentioned. Other clear air effects are mentioned briefly. Theoretical and experimental progress in the description of low elevation angle propagation is also included. Finally, a number of areas for further work are pointed out. It should be noted that the aspects of cross polarization are not included, as they are dealt with elsewhere.