Selenium (Se) belongs to the same group as sulfur in the periodic table but possesses certain chemical properties characteristic of a metal. It is an essential element in animals but becomes severely toxic when the amount ingested exceeds the required level. On the other hand, Se is not essential in plants although some plants are Se hyperaccumulators. Se changes into several chemical forms when metabolized. Thus, the identification of selenometabolites would enable us to formulate a metabolic chart of Se. Recently, speciation analysis by hyphenated techniques has contributed immensely to the study of selenometabolomes, i.e., the entirety of selenometabolites. Indeed, speciation has unveiled some unique selenometabolites in biological samples. The aim of this review is to present newly identified selenometabolites in animals and plants by speciation using hyphenated techniques and to delineate the perspectives of Se biology and toxicology from the viewpoint of speciation.