Selenium (Se) in geological samples can occur as selenide, selenite, and selenate with oxidation stages of: −2, +4, and +6, and Se often substitutes sulfur (S) in minerals. There is, however, limited information describing the occurrence as elemental Se (Se o ). In biological samples, Se may occur additionally in a number of biological molecules, such as the two amino acids selenocysteine (SeCys) or selenomethione (SeMet), which can be incorporated into proteins. While SeCys is an essential amino acid for most animals and forms selenoproteins such as glutathione peroxidase (GPx) or selenoprotein P (SelP). SeMet is only replacing cysteine (Cys) in Secontaining proteins and is not essential. Other environmental processes generate volatile Se species, such as dimethylselenide and dimethyldiselenide. More detail about the plethora of Se speciation is described elsewhere (Wallschlager & Feldmann 2010). Each group of Se species needs a bespoken methodology. In this mini-review, we are lining out the different types of analytical methods used to describe processes in three different case studies of Se.