Experiments on devices can be regarded as part of basic physics. Here, theoretical analyses of devices using the interface in a ferroelectric junction reveal novel basic properties unknown in conventional experiments. Devices using interfaces in ferroelectric junctions are grouped into two types: (1) the lateral-type, or field-effect, devices that use the transconductance, i.e., the conduction parallel to the interface; and (2) the vertical-type, or diode-like, devices, in which the conduction is perpendicular to the interface and the current flows through the ferroelectric. The discussion below concentrates on the field-effect type (1). On the other hand, the other type of devices have recently attracted intense interest due to the resistance RAM (R-RAM). Additionally, modulation of the tunneling current and the photovoltaic effect have been demonstrated. Details of these topics will be discussed elsewhere. We start our journey by assuming for the present that the ferroelectric is an ideal insulator.