Various organic conjugated materials have been used as active semiconducting layers in thin-film transistors. The mode of operation of these devices shows that a high carrier mobility together with a low conductivity are required. Conjugated polymers and other amorphous materials exhibit a low carrier mobility, of the order of 10-4−10-5 cm2V-1s-1, and all attempts to increase it through slight doping of the organic semioconductor have failed, owing to the existence of a direct relationship between mobility and conductivity. This behaviour can be understood from the variable range hopping mechanism which describes charge transport in these materials. On the other hand, conjugated oligomers are well–defined materials, offering various physical and chemical ways of controlling the structural organization of thin films made out of them. It is thus shown that carrier mobility is directly related to the long range structural order in these films, i.e. to the decrease of grain boundaries, leading to values close to 10-1 cm2 V-1 s-1, comparable to that of amorphous hydrogenated silicon. Besides which, conductivity in thin films of conjugated oligomers is mainly determined by the purity of the materials, allowing values lower than 10-7 S cm-1. This independent control of mobility and conductivity allows the realization of oligomer–based thin–film transistors showing characteristics close to those of classical a–Si:H–based thin–film transistors.