Bariatric surgery is increasingly being used to treat obesity. A distinction is made between procedures that are completely reversible (such as the insertion of a balloon into the patient's stomach) and operations that lead to irreversible changes (such as gastrointestinal bypass). However, not every person who is overweight is eligible for such surgery. What are the indications for bariatric surgery and who pays for the costs of such a procedure: can it be performed on the National Health Service or do patients have to pay for the treatment themselves?