Organic and polymeric materzials have seen a tremendous growth in research in the last five years as potential electroactive elements in thin-film-transistor (TFT) applications. These are driven by the increasing interest in flat-panel-display applications, for which organic and polymeric materials offer strong promise in terms of properties, processability, cost, and compatibility with eventual lightweight, flexible plastic displays. In this review we summarize the current status of our knowledge on the science of these organic and polymeric semiconducting materials. Most of these are based on linear thiophenes, especially a-hexathienyl, which has elicited by far the most attention. Mobility values in the 10−2–10−1 cm2/Vs and especially source-drain current on/off ratios of up to 106 make this a highly promising potential alternative to amorphous silicon. Other thienyl compounds are also discussed, as are polymeric analogues. A brief discussion of technological potential, limitations, and problems that need to be overcome is given at the end.