Selenium (Se) is an essential trace element, which is quite necessary for antioxidant defense and the regulation of thyroid hormone, among other related biological functions. Se is not only vital to human health, but also to plant growth; nevertheless, locations with high Se levels might produce toxic and contamination effects in the environment. Se comes from natural geological deposits, volcanic activity, agricultural runoff, and industrial emissions. Depending on the form, it can exist as selenite (SeO32−), selenate (SeO42−), and elemental Se, (Se0), which forms control its mobility and bioavailability. Se concentration, through bioaccumulation, affects aquatic systems and thus aquatic organisms to impact food web dynamics and ecosystem health. In the case of soils, Se adsorption, desorption, and leaching are controlled by properties such as texture, pH, and organic matter. Soil microorganisms play a highly significant role in Se cycling. While plant uptake mechanisms for Se may convey advantages to the plants, for instance, increased resistance to oxidative stress, at elevated exposures to high concentrations, similar uptake mechanisms result in toxicity. Ecological impacts of Se include effects on biodiversity and species interactions, evidenced through case studies of toxicity. Human health risks associated with Se include deficiencies and toxicities arising from accumulation along the food chain. The use of sustainable agricultural procedures and the employment of phytoremediation are some of the management strategies to be considered in Se-related problems, but there is also a need for regulatory frameworks to control emissions. Research gaps still exist in understanding Se dynamics, and interdisciplinary approaches are needed to enhance the knowledge and management of this essential element.