The technological demands for AR are much higher than for virtual environments or VR, which is why the field of AR took longer to mature than that of VR. However, the key components needed to build an AR system have remained the same since Ivan Sutherland’s pioneering work of the 1960s. Displays, trackers, and graphics computers and software remain essential in many AR experiences. Following the definition of AR step by step, this section first (describes display technologies) that combine the real and virtual worlds, followed by sensors and approaches to track user position and orientation for correct registration of the virtual with the real, and user interface technologies that allow real-time, 3D interaction. Finally some remaining AR requirements are discussed.