Onions (Allium cepa L.) and their by-products embrace significant medicinal, culinary, and industrial potential. This chapter investigates their nutritional profile, therapeutic value, domestic uses, and sustainable applications. Rich in macronutrients, micronutrients, and bioactive compounds such as flavonoids and sulfur compounds, onions hold anti-inflammatory, antimicrobial, cardiovascular, antidiabetic, and cancer-preventive effects. Furthermore, their prebiotic fibers support gut health. Beyond their therapeutic importance, onions enhance food flavor and preservation, while onion by-products serve as functional ingredients in dietary supplements, biodegradable packaging, and sustainable waste management. This chapter also explores recent advancements in onion research and identifies potential areas for future exploration. By integrating onions and their derivatives into nutrition, medicine, and eco-friendly applications, this chapter highlights their multifaceted role in promoting health and sustainability.