Discovered in 1818, selenium is a soft metalloid or semimetal. It is similar to sulfur in many ways, but overall it is more like tellurium. Selenium exists in several allotropic forms, therefore, its physical appearance varies from a grey metallic appearance to a red glassy appearance. The red allotrope is unstable and reverts to the grey form under normal conditions.

Selenium is usually found in the company of copper and sulfur. Virtually all commercial selenium is recovered from anode slimes in copper refineries and in the sludge that remains after producing sulfuric acid. These two highly profitable operations simply cast off selenium as a byproduct.

The most striking feature of selenium is its electrical conductivity. Unlike most electrical conductors, selenium varies in conductivity with variations in light. The conductivity of pure selenium can increase as much as a thousand-times when a sample is taken from darkness into bright sunlight. This "photoelectric" property accounts for the most common applications of the element and its compounds. Selenium can be found in all kinds of light-sensitive devices such as photocells, solar cells, television cameras and light meters. Selenium is also suitable for use in rectifiers.

Selenium has specialized applications in rubber vulcanization and catalysis and as a colourant in glass and ceramic products.