In low light, your subject's eyes are fully dilated to allow them to see better. When your flash goes off, the light reflects off the red retina in the back of the eye, giving the eye that signature demonic glow. Note that the only reason red eye happens is because the flash is so close to the camera lens--the light travels to the eye and reflects straight back, which gets caught by the lens. That means there are a few ways to avoid the red eye effect:

Digital SLR users often rely on that last option, since you can hold an external flash in your hand, mount it on a flash bracket, or even bounce it off a wall or ceiling. Read more about this in "."

Portraits that you take in close quarters can be ruined by the uneven lighting that comes from your flash, and really close-up photos can suffer from terrible overexposure and ugly shadows. There's a solution, though: a different sort of flash known as a ring flash.