Unlike a snail, a turtle is not able to trade in one shell for another if it's damaged or just doesn't fit anymore. The turtle's shell never falls off and is never too large or too small because it grows with the turtle. It's made from the turtle's rib cage and spine and is attached to the internal bones of the turtle's body. Just as your vertebrae grow with you, the same is true for a turtle's shell.
For most species, as the turtle and its shell grow, the scutes on the shell shed or peel away to make way for new, larger scutes. Shedding is a natural process, and scutes are cast off intermittently during daily activities such as swimming and basking. Since turtles are cold-blooded reptiles, they rely on external ways of heating and cooling themselves. Basking is nothing more than lying in the sun, and it happens to be a favorite turtle pastime.
While basking is one way turtles raise their body temperature, it helps them shed scutes by drying them up, leaving them ready to fall off. Some turtles, such as the South American river turtle lend each other a hand, well, specifically their jaw, in the shedding process by pulling loose scutes (and algae) off each other's shells [source: San Diego Zoo]. This is done gently, though, since both pain and pressure can be felt through the shell.
When old scutes aren't suitably shed or are shed too often, turtle shells can develop infection and disease. Dysecdysis, which is a fancy term for abnormal scute shedding, can cause infection. In rare instances, scutes are shed too frequently, leaving the bones of the shell unprotected and soft; abundant scute shedding has been linked to larger problems such as renal failure.
Shell health is also dependent on bone health. Metabolic bone disease, caused by inadequate calcium intake, poor exposure to sunlight, as well as diseases of the liver, kidneys and thyroid, can result in soft or misshapen shell bones. Ulcers (also known as shell rot), can cause permanent shell and scute deformities.
Shell disease doesn't occur as frequently as injury, however. Fractured shells are common and happen when turtles are hit by cars or attacked by wildlife. Some veterinarians are able to repair broken shells with bonding material, but one of the fantastic things about a turtle shell is that since it's made of living materials, it can slowly repair itself and regrow.
For more information about turtles and other reptiles, slowly peruse our list of resources on the following page.