Aircraft fly within the atmosphere and are wholly dependent on it for the generation of the aerodynamic forces that sustain and regulate flight. Aeroplanes are also propelled by air-breathing engines that ingest air from the atmosphere to support combustion and generate thrust. Further, all navigational and communication signals, including visual, must penetrate a layer of the atmosphere before reaching the aircraft. The same is true for signals from aircraft to ground-based facilities and controllers. The aviation process is therefore strongly affected by the state of the atmosphere. Atmospheric processes are very diverse in terms of their origin, physical nature, spatial and temporal scales, and intensity. However, from the point of view of effects on aviation, they may be classified into five different groups: (i) phenomena involving physical motion of air; (ii) hydrometeorological phenomena; (iii) phenomena inducing and facilitating ice formation on aircraft surfaces; (iv) phenomena causing low visibility; and (v) phenomena involving atmospheric electricity. The following sections discuss different facets of the complex interaction between weather and aviation, the characteristics of different types of atmospheric phenomena, and their effects on aviation.