Protein quality is a measure of how well a dietary protein meets the body’s needs for growth, maintenance, and repair. Two things decide it: the amino acid profile, judged against human requirements with attention to the limiting essential amino acids, and digestibility, meaning how much of the protein is actually absorbed. Animal proteins such as egg, milk, and meat score highly on both counts, while many plant proteins are limited in one or two essential amino acids, though combining sources like beans and rice compensates. Modern scoring systems, PDCAAS and its successor DIAAS, quantify quality by comparing digestible essential amino acid content with a reference pattern. Quality matters most where total protein intake is marginal, as in young children and restrictive diets.