Proteins are the polymers of Amino acids, i.e. a protein is a macromolecule built up by union of large number of amino acids in a linear chain. In a protein chain amino acids are linked by peptide linkage, hence called polypeptides.Proteins are essential constituent of protoplasm and principal building block of body. They also serve as an energy source in body. In a protein amino group (-NH) of one amino acid condenses with the carboxylic group (COOH) of another amino acid. Elementary composition of protein reveals that there is 54 % of carbon, hydrogen 7 %, nitrogen 16 %, oxygen 22 % at sulphur 1 % or phosphorus 0.6 %. On the basis of molecular composition, structure and origin proteins are classified as follows: Simple protein: Pure proteins without combined with anything else. Example: Protamins, Histones, Albumins. Conjugated protein: Chromoprotein, Haemoglobin, etc. Derived protein: Formed inside the living cell. Example: Proteose, peptones etc.