Inverter circuits and ring oscillators have been prepared based on the organic semiconductor pentacene. High-purity pentacene single crystals and thin films exhibit ambipolar transport properties, i.e., n- and p-channel transistor activity. This feature is very attractive for complementary logic electronic circuits, since only one organic material has to be used. Switching frequencies up to 11 MHz have been achieved for single crystal circuits. Thin film devices prepared on flexible polyimide substrates revealed delays of 0.1 μs per stage resulting in oscillation frequencies up to 900 kHz.