Cats' pupils get big (dilate) primarily to let in more light, which helps them see better in low-light or dark environments, as they are crepuscular animals active during dawn and dusk. Their pupils can also dilate when they are excited, playful, or hunting, as it helps them gather more visual information, or when they feel fear or anxiety as part of their fight-or-flight response. Additionally, pupil dilation can be caused by medical issues such as pain, high blood pressure, or other health problems, but most often it is a natural response to light and emotional stimuli.