Optical illusions are interesting within scientific disciplines because they lie on the border of what we are able to see. Different kinds of optical illusions may be better explained by one discipline or another. Some optical illusions trick us because of the properties of light and the way our eyes work, and are addressed by biology and perhaps optics, while other illusions depend on a “higher” level of processing which is better addressed by psychology. Some illusions can only be fully explained using knowledge of more than one discipline. When there are both biological and psychological explanations, they may both be right. In this paper, I cover some relevant theories from the fields of biology and psychology, and then apply them to four different optical illusions.