Technical Abstract: The aim of this review was to present and evaluate the present knowledge on which selenium species are available to the general population in form of food and common supplements and how these species are metabolized in mammals. The overview of the selenium sources is taking a horizontal approach, which encompass identification of new metabolites in yeast and food of plant and animal origin, while the survey on the mammalian metabolism is taking a horizontal as well as a vertical approach. The vertical approach encompasses studies on dynamic conversions of Se compounds within cells, tissues or whole organisms. New and improved sample preparation methods, separation and detection methods are evaluated from an analytical chemical perspective to cover the progress in horizontal speciation while the survey of the analytical methods for the vertical speciation and the interpretations of the results are evaluated from a biological angle as well.