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In aviation, icing conditions are those atmospheric conditions that can lead to the formation of water ice on the surfaces of the aircraft, or as carburetor icing within the engine. Inlet icing is another engine-related danger, often occurring in jet aircraft. These icing phenomena do not necessarily occur together. Many aircraft are not certified for flight into known icing — icing conditions which are certain to exist based on pilot reports, observations, and forecasts.

Ice forms when air containing water vapour is cooled below freezing point. Temperature decreases with pressure; ice may form wherever there is reduced pressure, including wings and empennage (where it is known as structural icing), carburetors, pitot tube, and engine inlets. Ice gradually builds as long as icing conditions exist, and if left unchecked results in dangerous conditions. Ice adds to an aircraft's weight and disrupts airflow, increasing the stall speed. In engines, carburetor ice and inlet ice can lead to reduced power or complete engine failure.

• Glaze ice is often clear and smooth. Supercooled water droplets, or freezing rain, strike a surface but do not freeze instantly. Often "horns" or protrusions are formed and project into the airflow.
• Rime ice is rough and opaque, formed by supercooled drops rapidly freezing on impact. Forming mostly along an airfoil's stagnation point, it generally conforms to the shape of the airfoil.
• Mixed ice is a combination of clear and rime ice.
• Runback ice is the result of water freezing on unprotected surfaces. Often forming behind deicing boots or heated leading edges, it was a factor in the crash of American Eagle Flight 4184.

Several methods exist to reduce the dangers of icing. The first, and simplest, is to avoid icing conditions altogether, but for many flights this is not practical.

If icing is expected, deicing fluid can be applied to the aircraft before flight to prevent ice from forming, or from sticking to the surface if it does form. Airliners are often deiced before departure, and the deicing fluid prevents ice buildup for a short time after.

Another common approach is to include heating elements along the leading edges of wings and tailplanes, which can keep the surface above freezing point. A weeping wing has hundreds of small holes in the leading edges and releases deicing fluid on demand to clear the entire wing of ice. Older aircraft and propellers are often equipped with pneumatic deicing boots

Carburetor heat is applied to carbureted engines to prevent and clear icing. Fuel-injected engines are not susceptible to carburetor icing, but can suffer from blocked inlets. In these engines an alternate air source is often available.