The purpose of this chapter is to introduce you to the human 3D perception. Our ability to combine the two images our brain receives from our eyes, to perceive depth, is called stereopsis. It’s just one of many ways we understand the 3D world we’re living in. We will see how stereopsis works and relates to other 3D perception tools, how it may sometimes not work properly, and, eventually, how it affects 3D moviemaking. The ﬁ nal experiment will give you a chance to voluntarily control your stereoscopic reﬂ exes.