This document provides an overview of three-dimensional displays, from physiological depth cues to electronic holograms. It discusses depth cues our eyes use to perceive depth, including psychological cues like occlusion and physiological cues like binocular disparity. Examples of 3D displays that provide some depth cues, like lenticular sheets, are described. The document also covers holograms, including how they can provide all depth cues by reconstructing the original wavefront. It discusses challenges like the large amount of information in holograms and methods to reduce it, like rainbow and multiplex holograms. Computer generated and electronic holograms using dynamic modulators are also summarized.