The United States Government approved the Law 600 authorizing Puerto Rico to draft it's own constitution in 1952. The Constitution was approved by a popular referendum and ratified by the U.S. Congress which maintains ultimate sovereignty over Puerto Rico, while giving Puerto Ricans a high degree of autonomy. Under this Constitution, Puerto Rico is a commonwealth associated with the United States.
The government is composed of 3 branches; the Executive branch, the Legislative branch with a bicameral Legislature consisting of a Senate and a House of Representatives and the Judicial branch. The legal system is based on the Spanish civil code.
See also: Politics of Puerto Rico