Cataracts are the leading cause of blindness worldwide. They can occur at any age, but most cataracts occur in people over 40 years old and are considered an age-related eye disease. A cataract is a clouding of the lens in the eye. The lens is a transparent disc located behind the pupil (dark opening in the center of the eye) and the iris (the distinctly colored part around the pupil). The lens helps to focus light on the retina, located at the back of the eye, where nerve signals are created and transmitted to the brain to create the visual image for what is seen. The lens is made of water, proteins, and polysaccharides. As we age, some of the proteins begin to clump together and the area in the lens associated with these proteins becomes cloudy. This results in a gradual change in the lens from transparent to yellowish brown. Light passing through a lens with a cataract is not clearly focused on the retina, so images received from the brain may be blurred or brown tinged. Cataracts may progress over a number of years and do not require treatment unless they compromise vision or prevent examination or treatment for other eye disorders. The May 20, 2009, issue of JAMA includes an article about cataract surgery.