The book focuses on the electrical properties and photoinduced processes in polymer based field-effect transistors PFETs. The photoinduced drain current in the PFETs results from a variety of possible pathways, and is a consequence of a complex charge generation, recombination and transport processes. These processes are observed to be dependent on the gate voltage and demonstrate a coupling between the photoexcitation and transistor features. Considerable increase in drain current upon irradiation, followed by an extremely slow relaxation upon terminating the illumination is the striking feature in poly(3-hexylthiophene) based transistor operated under accumulation mode. In depletion mode, the photoresponse becomes strongly dependent on gate voltage. Low dark current in the background, accompanied by large photosensitivity in this mode is exploited to observe memory effect including write, read, store and erase operations. The unique combination of gate bias and light as input parameters for charge transport in polymer transistor and the spatial information of the gate voltage dependent photocarrier generation region are thoroughly investigated using spectroscopic analysis.