Design and functional development of a user interface. Interface is a part or an act, in which a person makes contact with a machine or a product. The simplest example therefore is a light-switch: it‘s not part of the person, neither part of the machine (light), but an interface between. For being useful and important for persons, the interface should be based on human needs and skills. If you look at it systematically user interface belongs to the „Man Machine System“ (MMS): Man  ↔ Man-Machine-Interface ↔ Machine. Different sciences are focused on that theme, e.g. informatics, the Cognitive Science research and psychology. The basic knowledge for user-friendly interface design is developed in design and ergonomics. The specific field of activity is Cognitive Ergonomics, System Ergonomics and Software Ergonomics (usability engineering).