Optimization of the interface between the source/drain electrode and the organic semiconductor is one of the important factors for organic thin film transistor (OTFT) performance along with the insulator–semiconductor interface. In this Feature Article, items needed for the optimization of the metal–semiconductor interface and methods to measure interface properties are reviewed. Various electrode materials are compared in terms of the work function, contact resistance and the pentacene OTFT performance. The effect of surface modification is also discussed. Experimental data obtained in LAMP is introduced to show the effect of the various material properties more clearly.