This chapter discusses the phenomenon of visual discomfort in stereoscopic and multi-view 3D video reproduction. Distinctive definitions are provided for visual discomfort and visual fatigue. The sources of visual discomfort are evaluated in a qualitative and quantitative manner. Various technological influence factors, such as camera shootings, displays, and viewing conditions are considered, providing numerical limits for technical parameters when appropriate and available. Visual discomfort is strongly related to the displayed content and its properties, notably the spatiotemporal disparity distribution. Characterizing the influence of content properties requires well-controlled subjective experiments and a rigorous statistical analysis. An example of such a study for measuring the influence of 3D motion is presented in detail and important conclusions are drawn.