Sex change theory
The idea of sex change is based on the notion that a person of one gender can be born "trapped" in an opposite-gender body. The theory plays havoc with traditional ideas of masculinity and femininity.
Transgendered patients seek to undergo surgical sex change operations (also known as gender reassignment surgery) to change their external appearance to to that of the gender with which they identify. Patients often undergo psychotherapy, hormone treatment, and facial surgery as well. In the United States, a patient must undergo counseling before being approved for the surgery. Patients in the US are also required to have two letters of recommendation from certified psychiatrists, and must have lived a year acting as the gender that they identify with.  Patients sometimes choose to have the surgery done abroad in places like Thailand for three reasons: 1. It is often less expensive for the patient, 2. The patient may be able to bypass some legal restrictions, 3. The surgeons in Thailand have a reputation of being particularly skilled in this area. 
It should be noted that hormone therapy and removing and adding body parts does not change the genetic makeup of an individual. On the genetic level, individuals who started as men are still men no matter what appearance they choose and women who started as women are still women after surgery as well.
Different states have different laws regarding the status of people who have had sex change operations. Generally, their birth certificates are reissued with their new legal gender. Since they are not required to divorce before surgery, this could lead to de-facto same sex marriage if they stay with their spouse.
J. Michael Bailey wrote a book which disagrees with the "born trapped" theory.
- In his book, he argued that some people born male who want to cross genders are driven primarily by an erotic fascination with themselves as women. 
Dr. Bailey has come under tremendous social and administrative pressure from partisans who disagree with his scientific views. This pressure has focused primarily upon objections to the implications of his theory, rather than objections to his empirical evidence. Rather than try to refute his empirical findings, "transgender activists worked to try to ruin Bailey professionally and personally." 
According to the Vatican,
- people who have undergone a sex-change operation cannot enter into a valid marriage, either because they would be marrying someone of the same sex in the eyes of the church or because their mental state casts doubt on their ability to make and uphold their marriage vows. 
- Christine (born George) Jorgensen: An American World War II veteran, George became Christine in a 1952 operation. This was the first widely publicized sex change surgery.
- Walter Carlos, 1967
- Jan (born James) Morris: A British World War II veteran, reporter, and father, Morris underwent sex change surgery in 1972.
- Dr. Rene Richards (born Richard Raskin): An American doctor and tennis player. Richards made waves in 1976, when she entered amateur tennis competitions as a woman.
- "The key point is that the (transsexual) surgical operation is so superficial and external that it does not change the personality. If the person was male, he remains male. If she was female, she remains female." 
- Christine Jorgensen site
- Jan Morris: A Profile
- The Lady Regrets