The Master Thesis work presented here has been developed in order to study and characterise the performances of organic electrochemical transistors (OECT), fabricated by stereolithographic printing. OECTs are devices whose functional part is constituted by a semiconducting organic material. The general structure and the working principle are similar to traditional transistors, with some differences in the kind of charge which is transported in the channel. The physics of the device and a physical-mathematical model are thoroughly described in this work. An overview of the most common organic materials is presented. Nowadays the most commonly exploited material for this kind of devices is PEDOT, a conductive polymer which can be used as a filler inserted in an insulating polymeric matrix.