Proteolytic enzymes digest the protein to simple amino acids. These amino acids are absorbed through active transport mechanism. The absorbed amino acids are utilized to synthesize protein, undergo catabolism to generate non protein nitrogenous substances, urea, fatty acids, ketone bodies and so on. The catabolism of amino acids results in ammonia which is toxic to human body will convert to urea for excretion. Any defect with the urea cycle enzymes results in the accumulation ammonia or intermediates of urea cycle. Glycine is the simplest amino acid responsible for the production of heme, glutathione and purine nucleotides. Glutamate catabolism results in GABA and glutathione. Tyrosine can be synthesized by phenylalanine with the help of phenylalanine hydroxylase. The deficiency of this enzyme results in phenylketonuria. The tyrosine is responsible for the generation of DOPA, dopamine, melanin, epinephrine and norepinephrine. Homogentisate oxidase deficiency results in alkaptonuria with a symptom of dark urine and mental retardation. Serotonin, melatonin and niacin are the metabolic intermediates of tryptophan. The maple serum urine disease is the clinical condition arises due to the deficiency of a-keto acid dehydrogenase enzyme of branched chain amino acid metabolism. Homocystinuria is an inherited disorder of methionine metabolism. At the end of this chapter the students will understand about the digestion and absorption of proteins, metabolism of absorbed amino acids.