Proteins are important molecules in cells. Proteins are the major component of the dry weight of cells. Proteins are the most abundant intracellular macromolecules. Proteins are connected intimately with all chemical and physical activity, which constitutes the life of the cell. This chapter covers the chemical composition, structure of protein, properties of protein, classifications of protein, sources of protein, recommended dietary allowances, amino acids, biological value of protein, functions of protein, digestion of protein, and deficiencies of proteins. Proteins contain carbon, hydrogen, nitrogen and sulfur and some contain also phosphorus. Proteins are large molecules formed from the combination of a number of simpler substances known as amino acids. Biological value of protein is the percentage of protein nitrogen that is absorbed and available for use by the body for growth and maintenance.