The history of radar abounds with reports of strange echoes received from supposedly clear skies. Early observers suspected birds or stray weather balloons, but these were eliminated by visual checks. Conjecture that clouds of insects were responsible was also eliminated when such echoes were obtained in the dead of winter. Some connection with the weather was suspected after it was noted that echoes of this type became more numerous on summer nights under calm conditions. Additional evidence indicated that many of these echoes originated in the fine structures of the dielectric (refracting) layers of air-mass boundaries and in regions of air turbulence. Some of the sharpest echoes involved surfaces of pronounced transitions of the water-vapor content of the air. The bibliography at the end of this report contains numerous detailed references to these general phenomena.