From the viewpoint of dielectrics physics, electrostatic phenomena occurring at material interfaces is briefly described. The importance of charge storage due to Maxwell-Wagner effect and electrostatic energies stored at the interfaces is emphasized for analyzing the interfacial electronic phenomena. Analyzing of pentacene field effect transistors (FETs) as a Maxwell-Wagner effect element showed that the derived current equation well describes the I-V characteristics of pentacene FETs. It is also shown that injected carriers from source electrode are the origin of carriers of OFETs and form a space charge field in organic FETs (OFETs). Furthermore, it is demonstrated that the electric field induced optical second harmonic generation (EFISHG) method is a potential way to probe static electric field distribution along the FET channel. The method also enables us to directly visualize carrier motion in the OFET channel.