Proteins are polymers of amino acid. Carbon, hydrogen, oxygen, and nitrogen are the major components of protein. Nitrogen is characteristic of protein. All proteins are linked by peptide bond. An amide bond formed between an amino group and a carboxyl group at least one of which is not an alpha group. On an average, the nitrogen content of ordinary proteins is 16% by weight. This chapter also discusses the chemistry of proteins, structural organization of proteins, separatory techniques of proteins, precipitation reactions of proteins, methods of quantitation of total proteins, protein folding, glycoproteins, protein sorting and plasma proteins and immunoglobulins. Proteins are classified based on the shape of proteins, nutritional value, and composition which are also explained briefly in this chapter. Proteins have different levels of structural organization which is primary structure, secondary structure, tertiary structure, and quaternary structure. At the end of the chapter, multiple choice questions are also given along with their explanatory answers.