Protein quality, not just quantity, is critical for meeting human dietary needs. This paper reviews animal‑, plant‑, and emerging protein sources in terms of amino acid composition, digestibility, nutritional adequacy, and environmental impact. It highlights life‑stage–specific protein requirements, limitations of single‑source proteins, and the role of complementary proteins in balanced diets. Global patterns of protein availability, future demand, and sustainability challenges are examined. The paper concludes that diverse dietary protein sources, rather than exclusive reliance on plant or animal proteins, are essential for supporting human health and resilient food systems.