This chapter lays the foundations of operation of both unipolar and ambipolar organic thin film transistors (OTFTs). Thin film transistors are used in display back planes, digital circuits, memory addressing elements, and photodetection. The discussion describes basic transistor principles and the limitations to their performance. Transistor noise and circuit noise margin, cutoff frequency, transconductance, and gain are discussed. Several different lateral architectures including top and bottom gate, and split gate configurations, as well as vertical transistors are described. High performance OTFT materials and channel morphologies and how they are achieved along with contact patterning are discussed. Phototransistors are introduced, and their characteristics are compared with other photodetectors discussed in Chapter 7. Transistor stability and its implication for circuit performance are detailed. Finally, several circuit applications with particular focus on chemical and medical sensing, and communications are described.