Visual discomfort is a significant obstacle to the wider use of stereoscopic 3D displays. Many studies have identified the most common causes of discomfort, and a rich body of literature has emerged in recent years with proposed technological and algorithmic solutions. In this paper, we present the first comprehensive review of available image processing methods for reducing discomfort in stereoscopic images and videos. This review covers improved acquisition, disparity re-mapping, adaptive blur, crosstalk cancellation and motion adaptation, as well as improvements in display technology.