An optical illusion represents a perception that deviates from the actual visual information of the visualized scene and is considered to be generated by an error in visual processing in the brain. The mechanisms by which optical illusions are generated have attracted the interest of scientists for many years. In this review, I focus on a vertebrate model, the zebrafish, and discuss how research using zebrafish has uncovered the mechanisms underlying optical illusions, and how optical illusions serve as a tool to help us understand how normal visual processing works in the brain.