An overview of recent work on the connection between electrical and molecular properties of semiconducting polymers for thin-film transistors (TFTs) is presented. A description of the molecular packing and microstructure of amorphous to semicrystalline semiconducting polymers is presented. The features of basic models for electrical transport in TFTs are discussed. These studies indicate that defect states and traps are as important as ordered domains for understanding transport in semiconducting polymers. Advanced methods, such as electric force microscopy, useful for measuring the characteristics of defect states and charge traps, are briefly reviewed.