Selenium has a biological role, and is found in organic compounds such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine. In these compounds selenium plays an analogous role to sulfur.

Diphenyl diselenide is the chemical compound with the formula (C₆H₅)₂Se₂, abbreviated Ph₂Se₂ This orange-coloured solid is the oxidized derivative of benzeneselenol. It is used as a source of the PhSe unit in organic synthesis.

Selenium occurs naturally in a number of inorganic forms, including selenide, selenate and selenite. In soils, selenium most often occurs in soluble forms like selenate (analogous to sulfate), which are leached into rivers very easily by runoff.

Selenium (pronounced /səˈliːniəm/) is a chemical element with the atomic number 34, represented by the chemical symbol Se. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature.

Isolated selenium occurs in several different forms, the most stable of which is a dense purplish-gray semi-metal (semiconductor) form that is structurally a trigonal polymer chain. It conducts electricity better in the light than in the dark, and is used in photocells (see allotropic section below).

Selenium is found in economic quantities in sulfide ores such as pyrite, partially replacing the sulfur in the ore matrix. Minerals that are selenide or selenate compounds are also known, but all are rare.