A user interface is the part of the system with which the users interact. From the user's point of view, the user interface is the system. It includes the screen displays that provide navigation through the system, the screens and forms that capture data, and the reports that the system produces (whether on paper, on the screen, or via some other medium). This chapter introduces the basic principles and processes of interface design and discusses how to design the interface structure and standards, navigation design, input design, and output design. The chapter introduces the issues related to designing user interfaces for the mobile computing environment and social media. It also introduces the issues that need to be considered when designing user interfaces for a global audience. Finally, the chapter describes the affect of the nonfunctional requirements on designing the human–computer interaction layer.