The polyanilines are a class of polymers the base form of which has the general formula [graphic omitted] containing y reduced and (1 –y) oxidized repeat groups. y can in principle be varied continuously from one, the completely reduced material, to zero to give the completely oxidized polymer. The emeraldine oxidation state (y= 0.5) consists of alternating reduced and oxidized groups. It can be protonated, i.e. doped, by aqueous acids with a concomitant increase in conductivity of almost 10 orders of magnitude (to a maximum conductivity of 10¹–10² S cm^–1), forming a polysemiquinone radical cation such as [graphic omitted] which contains a delocalized half-filled broad polaron energy band. The polymer is readily solution-processed into films and fibres which can be mechanically aligned, the doped forms of which have a conductivity parallel to the direction of alignment significantly greater than that of non-aligned material. X-Ray studies show that the doped and undoped polymer exist in several different crystalline forms. A wide variety of derivatives can be synthesized by substitution on the ring or on the nitrogen.