Organic transistors and circuits are technologically interesting because they have potential to serve in inexpensive and flexible electronic circuits. Major applications include radio frequency identification tags and flexible display backplanes. These circuits can be potentially fabricated by simple printing methods, not requiring the demanding environment needed for silicon based circuitry. This paper reviews the principles of transistors and discusses the issues most important to organic transistors. It also covers the fabrication and properties of n-channel devices as well as ambipolar transistors.