This is when a person sees double … that is to say, they see the same object twice. This can be a horizontal side-by-side doubling, a vertical up-down effect, or a combination of the two. Whenever a person has “double vision” the biggest question we ask is whether this is a monocular (one-eyed) diplopia or a binocular (two-eyed) problem. If the doubling goes away when covering either eye, then we know this is a binocular problem where the eyes are out of ‘sync’ with each other (such as with crossed-eyes). There are many potential causes for binocular diplopia such as a cranial nerve palsy or a stroke. This alignment problem requires a thorough workup and may involve an MRI and neurologic evaluation depending upon our findings. If the double vision persists for many months, prism glasses or strabismus surgery may be required to get the eyes straightened out. Monocular (one-eye) diplopia is simpler and less concerning – if double vision remains, even when the other eye is covered, than we know the problem is located in just the eyeball itself. Monocular diplopia is rarely an emergency and can be caused by astigmatism, dry eye, cataracts, and rarely retinal problems like an epiretinal membrane.