We have fabricated the fastest organic circuits on flexible substrates yet reported. These circuits use the small-molecule hydrocarbon pentacene as the active semiconductor material and 75 mum thick flexible, transparent, colorless, polyethylene naphthalate (PEN) film as the substrate. Transistor arrays, inverters, ring oscillators, and other circuits with good electrical performance, yield, and uniformity were obtained. A field-effect mobility of: 1 cm(2) / V-s was extracted from OTFT saturation characteristics, and ring oscillators had minimum propagation delay < 40 <mu>sec per stage and < 50 <mu>sec per stage at bias levels below 8 V.