The performance of organic field-effect transistors (OFETs) has improved substantially in recent years. Their field-effect mobilities are now comparable to that of hydrogenated amorphous silicon thin-film transistors (TFTs). Both TFTs and OFETs operate in the accumulation regime and the authors here develop a comprehensive model for OFETs in this regime. They show that, in the case of a constant mobility, a threshold voltage cannot be defined. The threshold voltage observed in practice is attributed to a gate-voltage-dependent mobility.