Selenium in the Environment and Human Health explores one of the most fascinating trace elements at the interface of environmental science, nutrition, molecular biology, and medicine. Selenium is essential for human life, yet excessive exposure can be harmful, creating a narrow biological window that requires careful scientific interpretation. This volume presents an integrated overview of selenium distribution, bioavailability, redox biology, selenoprotein function, immune regulation, oxidative stress, nano-enabled therapeutic perspectives, and disease-related outcomes. By connecting environmental pathways with molecular mechanisms and clinical relevance, the book highlights why selenium status must be understood in relation to chemical form, dose, geography, diet, and individual vulnerability. It provides valuable insights into selenium deficiency, toxicity, risk assessment, and the potential implications of selenium for cardiovascular, metabolic, thyroid, immune, reproductive, and neurodegenerative conditions. Designed for researchers, clinicians, environmental professionals, nutrition specialists, and health scientists, this volume offers a multidisciplinary reference for understanding how selenium contributes to both environmental integrity and human well-being.