The need for gender reassignment surgery stems from the feeling that the physical body does not match the brain, or the way the patient feels. The patient may feel like a woman mentally, but be physically a man. Surgery is done to help make the patient's physical body match the way they feel.
For females, the surgery may include a mastectomy and a hysterectomy, but these surgeries are not necessary in all cases.
Psychological testing is a component of the surgery, and patients must undergo rigorous testing to make sure they can tolerate the stresses of transforming from one gender to another. Most surgical centers require patients to live as the gender they will be changing to for no less than one year prior to surgery.