Most amino acids in the body are alpha amino acids. Amino acids can be classified based on their (i) structure (ii) metabolic fate (iii) nutritional requirements. In solution, amino acids exist as ‘Zwitter ions’ or ‘Ampholytes’ at their characteristic isoelectric pH (pI). In this state they carry no net charge. Glycine has no asymmetric carbon atoms and therefore has no optical activity. Alpha carboxyl group of one amino acid combines with the alpha amino group of another amino acid to form a peptide bond. Proteins are polymers of amino acids linked adjacently by peptide bonds. Nitrogen content of ordinary proteins is on the average 16 % by weight. Protein structure can be defined and studied at four levels viz. primary, secondary, tertiary and quaternary all proteins have a N-terminal (amino) and a C-terminal (carboxy). Cysteine forms disulfide linkages between two polypeptide chains in oligomeric proteins. Primary structure determines the biological activity of the protein. Alterations lead to loss of functional capacity, e.g., Sickle cell hemoglobin (HbS). Secondary, tertiary and quaternary structures of proteins are stabilized by hydrogen bonds, ionic bonds, hydrophobic interactions and van der Waals forces. Secondary structure could be an alpha-helix or a beta-pleated sheet. Examples of oligomeric proteins with quaternary structure are hemoglobin, myoglobin and creatine kinase. Solubility of a protein is dependent on the ionic concentration of the medium. Hence, proteins may be ‘salted out’. Denaturation of proteins results in loss of biological activity but not the primary structure. Denaturation may be reversible. Proteins can be classified based on: (i) functions, (ii) composition, and (iii) nutritional value.