Abstract: π-Conjugated organic molecules are capable of transporting charge and interact efficiently with light similar to inorganic materials. Therefore, these systems can act as semiconductors in optoelectronic devices. However, organic chemistry offers the advantage of tailoring material functional properties via modifications of the molecular core units, opening new exciting possibilities for inexpensive device manufacture. This chapter reviews fundamental aspects behind the structural design/realization of molecular semiconductors for organic field-effect transistors (OFET). An introduction to the basic principles of OFET and the state-of-the-art organic molecular semiconductors for organic transistors is presented.