The document discusses the history and principles of natural user interfaces (NUIs). It notes that Bill Gates predicted in 2008 that interaction with computers would dramatically change within five years to involve touch-based interfaces. The document outlines key characteristics of NUIs, such as being effectively invisible, intuitive with little or no learning curve, and supporting natural gestures. It discusses Microsoft's Metro and Modern UI design philosophies for clean, minimalist interfaces focused on typography and content. The document also covers PixelSense interfaces and their 360-degree, multi-user touch capabilities.