Selenium (Se) is an essential trace element crucial for human health, playing a key role in the function of selenoproteins, which are involved in antioxidant protection, redox regulation, and energy production. Adequate Se intake is necessary to support the maximal expression of Se-containing enzymes, with recommended intakes of at least 40 μg/day to maintain health and up to 300 μg/day to reduce cancer risk. However, many populations worldwide have low Se intakes, leading to impaired antioxidant defenses and increased susceptibility to oxidative stress, infections, and chronic diseases. Supranutritional Se intakes, while potentially beneficial for cancer prevention, require careful management to avoid toxicity. Se is primarily obtained from soil, which varies widely in Se content, affecting the Se levels in food systems. Low Se soils result in low Se in plants, livestock, and humans, contributing to Se deficiency. Conversely, high Se soils can lead to Se toxicity (selenosis). Se deficiency is associated with diseases such as Keshan disease (a severe cardiomyopathy) and Kaschin-Beck disease (a chondrodystrophy), both prevalent in regions with low Se availability. Se supplementation has been effective in preventing and treating these conditions. Se is incorporated into proteins as selenocysteine (SeCys) through a complex process involving tRNA modification. Se is found in various food sources, with cereals, meats, and fish being the primary contributors. Se bioavailability varies depending on the form and source of Se in the diet. Se-enriched foods, such as Se-fortified salt, yeast, and certain vegetables, have been developed to increase Se intake and reduce deficiency. Se deficiency is widespread, with many populations not meeting the recommended intake levels. Low Se status is linked to increased risks of various diseases, including cardiovascular issues, infections, and cancer. Supranutritional Se intakes, as seen in some regions, have been associated with reduced cancer risk, but careful monitoring is needed to prevent toxicity. To address Se deficiency, strategies such as Se fertilization of crops, Se-enriched foods, and dietary supplements are being implemented. These approaches aim to increase Se intake to support health and reduce the burden of Se-related diseases. However, further research is needed to optimize Se bioavailability and ensure safe levels of intake. Overall, Se is a critical nutrient for human health, and improving Se status in food systems is essential for public health.Selenium (Se) is an essential trace element crucial for human health, playing a key role in the function of selenoproteins, which are involved in antioxidant protection, redox regulation, and energy production. Adequate Se intake is necessary to support the maximal expression of Se-containing enzymes, with recommended intakes of at least 40 μg/day to maintain health and up to 300 μg/day to reduce cancer risk. However, many populations worldwide have low Se intakes, leading to impaired antioxidant defenses and increased susceptibility to oxidative stress, infections, and chronic diseases. Supranutritional Se intakes, while potentially beneficial for cancer prevention, require careful management to avoid toxicity. Se is primarily obtained from soil, which varies widely in Se content, affecting the Se levels in food systems. Low Se soils result in low Se in plants, livestock, and humans, contributing to Se deficiency. Conversely, high Se soils can lead to Se toxicity (selenosis). Se deficiency is associated with diseases such as Keshan disease (a severe cardiomyopathy) and Kaschin-Beck disease (a chondrodystrophy), both prevalent in regions with low Se availability. Se supplementation has been effective in preventing and treating these conditions. Se is incorporated into proteins as selenocysteine (SeCys) through a complex process involving tRNA modification. Se is found in various food sources, with cereals, meats, and fish being the primary contributors. Se bioavailability varies depending on the form and source of Se in the diet. Se-enriched foods, such as Se-fortified salt, yeast, and certain vegetables, have been developed to increase Se intake and reduce deficiency. Se deficiency is widespread, with many populations not meeting the recommended intake levels. Low Se status is linked to increased risks of various diseases, including cardiovascular issues, infections, and cancer. Supranutritional Se intakes, as seen in some regions, have been associated with reduced cancer risk, but careful monitoring is needed to prevent toxicity. To address Se deficiency, strategies such as Se fertilization of crops, Se-enriched foods, and dietary supplements are being implemented. These approaches aim to increase Se intake to support health and reduce the burden of Se-related diseases. However, further research is needed to optimize Se bioavailability and ensure safe levels of intake. Overall, Se is a critical nutrient for human health, and improving Se status in food systems is essential for public health.