This chapter describes the structure of a group of polymers responsible for regulating the basic functions and inheritance of living organisms: the proteins. The characterization of these organic units involves a very complex problem due to its multi-factorial nature. To develop this topic I showed the difficulty when trying to differentiate living matter from non-living matter and then I proceeded to find distinctions at cellular level. At this stage I identified two main groups with three types of major macromolecular structures, where proteins are included, and four levels of structural complexity known as primary, secondary, tertiary and quaternary structures, from which proteins get their morphologic features.
Keywords: α-amino acids, Amide group, Eukaryotes, Carboxyl group, Covalent bond, Gene, Ionic bond, Macromolecules, Monomers, Non covalent bond, Nucleic acids, Peptide bond, Polymers, Polymerization, Polysaccharides, Prokaryotes, Proteins, Van der Waals` forces.