A cataract is a clouding of the normally clear lens of the eye with the amount and pattern of cloudiness varying. If the cloudiness is not near the center of the lens, you may not be aware that a cataract is present. Cataracts can be compared to a window that is frosted or fogged with steam and results in blurred vision. Cataracts occur inside the eye, not on the surface, and they can develop in one or both eyes. How quickly the cataract develops varies among individuals, and may vary even between the two eyes. Most cataracts associated with aging progress gradually over a period of several years. Other cataracts, especially in younger people and people with diabetes, may progress rapidly over a few months. It is not possible to predict exactly how fast cataracts will develop in any given person. When vision loss due to cataracts affects a person’s ability to do the tasks they want to do or need to do, cataract surgery becomes necessary. It is the leading cause of blindness in the world and the most common surgery performed with over 2.5 million people a year having cataract surgery in the United States.