|A review of characterization methods used to study semiconducting polymers in thin film transistors is presented. The basic operation of polymeric thin film transistors (TFTs) is described. Methods for microstructural characterization of thin films of semiconducting polymers such as x-ray scattering are discussed. Examples of how these methods have been applied to study structural ordering in semicrystalline polymers such as poly(3-hexylthiophene) are presented. The electronic structure of these materials is important for understanding the operation of TFTs and has been studied using ultraviolet photoelectronic spectroscopy, optical spectroscopy, and electrochemical methods. Instabilities of the operation of TFTs have been examined using time-dependent current-voltage measurements, charge modulated spectroscopies, and scanning probe methods. These methods have revealed the nature of charge carriers and trap states in semiconducting polymers. (C) 2008 American Vacuum Society.