Selenium (Se), a metalloid with properties intermediate between metals and non-metals, is regarded as a micronutrient vital for physiological activities, with a limited range in which it is deemed essential. The natural occurrence of Se in the environment includes geological deposits and plant absorption, alongside anthropogenic sources such as agricultural runoff of arid, mining activities, soils, disposal of fossil fuel byproducts, industrial processes such as coal combustion and metal refinement, and petroleum processing facilities. These activities may increase Se levels in aquatic and terrestrial ecosystems, potentially resulting in environmental risks. Se is essential for human health as it protects against oxidative stress and enhances immunological function. It also enhances quality, growth, stress resilience and yield in plants. The margin between advantageous and detrimental amounts of Se is exceedingly narrow, presenting issues for environmental management. At low concentrations, it functions as a micronutrient, enhancing plant growth and resilience to stress. At elevated quantities, it may be hazardous to plants, resulting in stunted development and reduced yields. High levels of Se exposure can induce malformations and reproductive failures in aquatic life-forms. Se has the capacity to bioaccumulate in food chains, resulting in biomagnification. Hence, comprehending these effects is essential for agricultural practices and food safety, as well as for formulating environmental policies and regulations. This chapter emphasizes the critical significance of Se for both flora and fauna, while also outlining its potential toxicity in excessive amounts. Thus, this chapter highlights the intricate relationship between the advantages and hazards of Se in the environment, emphasizing the necessity for a careful equilibrium between its vital roles and the safeguarding of ecosystems and human health.