The performance of organic based field-effect transistors (OFETs) has considerably increased recently, and now compares to that of amorphous silicon. The operation mode of OFETs is described. It differs from that of the conventional MISFET in that it operates in the accumulation regime. Another important difference is the charge transport in organic materials, which occurs via hopping of self-localized polarons. As a result, the mobility is thermally activated and electric field dependent. Effects of this feature on the current-voltage characteristics are outlined.