The thesis reports on the development of organic semiconductors for p- and n-type accumulation mode organic electrochemical transistors (OECTs). First, the development of a series of alkoxy benzodithiophene (alkoxy-BDT) copolymers is presented. The materials series were prepared and characterised for their electrochemical redox activity in aqueous electrolytes. In addition, the materials were tested in OECTs where the performance differences between the copolymers were related to their microstructure and redox stability. After the successful development of p-type OECT materials, the redox stability of alkoxy-BDT copolymers was investigated, where a strong dependence of the electrochemical redox stability was observed. It was found that alkoxy-BDT copolymers with a large ionisation potential (IP) formed a quinone side product whilst copolymers with small IPs were found to be redox stable. The formation of the quinone structure affects the performance copolymers in OECTs significantly and the formation of the quinone must be avoided for use of materials in OECTs. Based on the results of the alkoxy-BDT copolymer study, a new copolymer was developed showing a high performance in accumulation mode OECTs. It was shown that the choice of the side chain is highly important to facilitate the copolymer with a high ion mobility. Finally, the development of the first accumulation mode ambipolar OECT material is reported. The design strategy for the polymer is presented and it was observed that a large electron affinity (EA) is needed to operate the n-type polymer with a high stability. This demonstration opens the possibility to develop complementary circuits and enable sensing of biomolecules with the aid of enzymatic reactions.