This chapter discusses proteins, which are large molecules, their basic units being alpha-amino acids. Proteins are one of the three macronutrients which can yield energy in the body. Protein requirement is relatively higher in growing children, during pregnancy and lactation, and during convalescence. The large variety makes it possible for proteins to play highly specific roles as enzymes, antigens, antibodies, hormones and receptors. Essential amino acids must be supplied by the diet in quantities commensurate with their requirements for satisfactory nutrition. Protein of plant origin, commonly called vegetable protein, is obtained mainly from cereals and pulses. Dietary sources of animal protein include milk and milk products, egg, fish and other flesh foods. Combination of cereals and pulses can provide a reasonably balanced combination of all essential amino acids required by the human body. Protein is a unique nutrient in that its quantitative requirement depends partly on the quality of dietary protein.