We have fabricated pentacene organic thin film transistors (TFTs) on flexible polyester substrates with good yield and uniformity. These transistors have excellent electrical characteristics, with carrier field-effect mobility as large as 0.9 cm(2)/V s, on/off current ratio of 10(6), subthreshold slope of 1 V/decade, and near-zero threshold voltage. We have applied a simple model to estimate the resistance of the source and drain contacts in these transistors and found that the contact resistance is typically greater than the channel resistance. This suggests that the electrical performance of organic TFTs, in which reliable contact doping is difficult, may be dictated by the contacts, rather than by the intrinsic carrier mobility of the organic semiconductor. (C) 2002 Elsevier Science Ltd. All rights reserved.