Selenium is an important micronutrient in humans and animals, despite its toxicity at high concentrations. This first chapter performs a thorough review of selenium bioavailability in food stuffs and human diet. The second chapter reviews the selenium essentiality and toxicity, the aspects related to the selenium dietary supplementation and its daily consumption. The third chapter describes the status of selenium and their metabolites in an antiretroviral therapy context. The next chapter looks at selenium supplementation and autoimmune thyroid diseases. Next, the authors address the association between diabetes type-2 and its levels in serum related to nutritional habits. Then, the preparation of nanosystems based on zero-valent selenium nanoparticles since this essential trace elements carries unique characteristics is explored. Finally, the prospects of using nanoparticles of elementary selenium as a nutrition source are discussed.