In the years since the discovery of conductivity in polyacetylene, a tremendous amount of research has been devoted to the synthesis, characterization, and application development of electrically active polymers. This article reviews the theory of conduction in these materials and discusses the syntheses of the major types of electrically active polymers. Electrical conductivities of these polymers are presented along with stability and mechanical and optical properties. The unique combinations of properties in these materials have led to a wide range of practical applications, which are also discussed.