Sunglasses or sun glasses (informally called shades or sunnies; more names below) are a form of protective eyewear designed primarily to prevent bright sunlight and high-energy visible light from damaging or discomforting the eyes. They can sometimes also function as a visual aid, as variously termed spectacles or glasses exist, featuring lenses that are colored, polarized or darkened. In the early 20th century, they were also known as sun cheaters (cheaters then being an American slang term for glasses).

Since the 1930s, sunglasses have been a popular fashion accessory, especially on the beach.

The American Optometric Association recommends wearing sunglasses that block ultraviolet radiation (UV) whenever a person is in the sunlight to protect the eyes from UV and blue light, which can cause several serious eye problems. Their usage is mandatory immediately after some surgical procedures, such as LASIK, and recommended for a certain time period in dusty areas, when leaving the house and in front of a TV screen or computer monitor after LASEK. Dark glasses that do not block UV radiation can be more damaging to the eyes than not wearing eye protection at all, because they tend to open the pupil and allow more UV rays into the eye.

Since the 13th century and until the spread of contemporary UV-shielding spectacles against snowblindness, Inuit made and wore snow goggles of flattened walrus or caribou ivory with narrow slits to look through to block almost all of the harmful reflected rays of the sun. In many different forms and with many different materials, the indigenous peoples of North America and northern Asia crafted highly efficient equipment to protect their eyes against the damaging effects of strong sunlight in icy circumstances.

Pliny the Elder claimed that the Roman emperor Nero liked to watch gladiator fights using cut emeralds. These, however, appear to have worked rather like mirrors.

The first sunglasses, made from flat panes of smoky quartz called Ai Tai (靉靆), meaning "dark clouds," which offered no corrective powers but did protect the eyes from glare, were used in China in the 12th century or possibly earlier. Documents describe the use of such crystal sunglasses by judges in ancient Chinese courts to conceal their facial expressions while questioning witnesses.

In 1459, Nuno Fernandes made a request for a pair of spectacles to protect the eyes while horseriding in the snow against the glare coming from the snow, though no description of any actual spectacles is given.

King Louis XIV's court watched the 1706 solar eclipse through a telescope with a smoky glass filter attached.