Conjugated polymers as a special category of electronic polymers have received a tremendous amount of attention in the past decades. This chapter starts with a brief history of conjugated polymers and then discusses the fundamental difference between metals, semimetals, and semiconductors. Most conjugated polymers (if undoped) are semiconductors; however, the charge transport mechanism in conjugated polymers is significantly different from that in the inorganic semiconductor, which is elaborated next. Much of the discussion is dedicated to doping, commonly used to increase the number of charge carriers in conjugated polymers, which can lead to appreciable conductivity when coupled with high carrier mobility. Lastly, common methods to measure the charge carrier mobility are introduced and discussed.