Sun tanning or simply tanning is the process whereby skin color is darkened or tanned. The process is most often a result of exposure to ultraviolet (UV) radiation from the sun or from artificial sources, such as a tanning bed, but can also be a result of windburn or reflected light. Many people deliberately tan their skin by exposure to the sun, called sun bathing, or by the use of artificial tanning methods. Some people use chemical products which can produce a tanning result without exposure to ultraviolet radiation. Casual exposure to the sun has moderate beneficial impact, including the production of vitamin D by the body; but excessive exposure to ultraviolet rays has detrimental health effects, including possible sunburn and even skin cancer as well as depressed immune system function and increased risk of accelerated aging. To avoid sunburn, most people apply suitable sunscreen to skin exposed to the sun, but others use oils to accelerate the tanning process.

Some people tan or sunburn more easily than others. This may be the result of different skin types and natural skin color, and these may be as a result of genetics.

The term "tanning" has a cultural origin, arising from the color tan. Its origin lays in the Western culture of Europe when it became fashionable for young white ladies to seek a less pale complexion (see Cultural history below).