Visual illusions are distortions, alterations, or alternatives in the appearance of reality that result primarily from the sensory and perceptual processing mechanisms of the human visual system. These illusions are common phenomena in radiology, and it is important to understand them because they can create the illusion of disease, leading to incorrect image interpretation. Visual illusions in radiology can be organized according to the point in the visual system at which they emerge. Illusions of sensation, including Mach bands and background effect, are "lower-order" visual phenomena that occur as the eye and brain translate light into neural activity. Illusions of perception, such as ambiguous figures and distortion, occur in "higher-order" brain structures that are responsible for coalescing sensory input into a mental image interpreted by the mind. Illusions of image formation (eg, parallax), as the name implies, result from the way images are generated. Some visual illusions occur with all modalities, whereas others tend to be modality specific. The authors discuss a variety of visual illusions, describing their underlying psychologic and neuroscientific basis and demonstrating their relevance to radiology. A thorough understanding of visual illusions in radiology enhances diagnostic accuracy by minimizing the risk of mistaking illusions for disease.