This is formed, in the ideal situation, when two monomers in a reaction mixture add consecutively to create a polymer chain with a regular {ABABAB} structure, irrespective of the monomer feed ratio. When this happens the resulting copolymer will always have the same composition, a feature which can be advantageous but also somewhat restrictive, as the ability to vary the properties is then limited. Within a series entitled Speciality Polymers it seems appropriate then to deal with this subject, particularly as no previous attempt has been made to draw together the various facets of alternating copolymerization into one volume. It also seems timely to present a more unified picture of the subject which will also illustrate the progress made.