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Nowadays, there is a growing interest in studies on various types of biological activity, toxicity, and usage of both nonorganic and organic forms of selenium. It seems clear that such high scientific interest exists because maintaining the physiological level of selenium in the body is vital. The main biological role of selenium involves being a cofactor unit of selenium‐containing enzymes (Dumitrescu and Refetoff 2011). It needs to be noted that those enzymes are among the main ones in the functioning of the redox system of the cell and, thus, all the basic parameters of cell vital functions depend on their activity (Huang et al. 2012). It is found that action of selenium‐dependent enzymes in tissues, deiodinase and glutathione peroxidase, directly depend on the selenium intake in the body (Villette et al. 1998).

Naturally, selenium enters the human and animal body mainly in the form of selenium‐containing amino acids (Gammelgaard et al. 2011). At the same time, there are only a few reports available on biological activity of selenium in a nanosized form than on organic and nonorganic selenium compounds. Particularly, it is observed that red nanoselenium is less toxic and more biologically active than other nonorganic (Zhang et al. 2001; Sadeghian et al. 2012) and organic forms of selenium (Wang et al. 2007; Zhang et al. 2008).

In Caco‐2 cell line model it is established that intracellular transport of selenium depends on its chemical form. The lowest speed was documented for sodium selenite while selenomethionine and nanoselenium did not differ for this indicator (Wang and Fu 2012). To date, there is no consensus about the effect of the size of selenium nanoparticles (SeNPs) on its biological activity. Thus, activation of selenium‐dependent enzyme systems in mouse liver and human hepatoma cell line HepG2 does not depend on the size of SeNPs (Zhang et al. 2004). Meanwhile, Peng et al. (2007) showed that smaller size SeNPs (36 nm) had more biological activity than bigger ones (90 nm) (Peng et al. 2007).

Nowadays, different fields of application of selenium in nanoform are studied. A large number of studies are dedicated to the use of SeNPs for diagnosis and treatment of various diseases. In addition, their use as drug delivery systems has been also studied. Considering these facts, the present chapter is focused on the analysis of known data and the prospects of selenium nanocomposites used for biomedical purposes.