Time-varying patterns such as flickering lights can cause discomfort and induce seizures in photosensitive observers. An understanding of the temporal characteristics of visual discomfort is therefore important from both scientific and practical viewpoints. The purpose of this paper is to review existing studies on the impact of temporal characteristics on discomfort. Two related factors have been suggested as predictors of discomfort caused by time-varying patterns: (1) excessive contrast energy at the medium temporal frequencies to which the visual system is generally most sensitive, and (2) temporal deviations from the natural (1/f) statistical characteristic. These effects mirror the visual discomfort caused by spatial patterns, in some ways but not all. The interaction between spatial and temporal parameters needs to be investigated to clarify the factors underlying visual discomfort.