This chapter describes the composition and metabolism of amino acids and proteins. Proteins contain carbon, hydrogen, nitrogen and sulfur as major constituents. Some proteins also contain phosphorus as the chief constituent. Proteins are present as the main constituent of all living matter. Proteins are among the central molecules of biology. Proteins transmit and commute signals from the external environment to cell interior, duplicate genetic information, transform the energy in light, carryout chemical reactions with tremendous efficiency, transport molecules between cell compartments, etc. This chapter covers the importance of protein, general properties of protein, classifications of protein, color reaction of proteins, amino acid, classifications of amino acids, properties of amino acids, functions of amino acids, digestion and absorption of proteins, mechanism of amino acid absorption, biosynthesis of protein, protein metabolism, urea cycle, formation of niacin, formation of melanin, formation of thyroid hormone, formation of catecholamines, protein metabolism disorders, and nonprotein nitrogenous substances.