During the past three decades there have been major improvements in the techniques for isolating and characterizing proteins. As a consequence of the application of these improved procedures it has become apparent that proteins are not simple linear polymers of amino acids but have three–dimensional structure. Many proteins consist of several component peptide chains and they may contain in addition to amino acids other associated components such as metals, carbohydrates, various prosthetic groups as well as substituted amino acids. Proteins are often segregated into specific cellular organelles.