Puerto Rico was annexed by the United States from Spain in 1898, with Puerto Ricans granted U.S. citizenship by birth in 1917. As a result, Puerto Ricans in the mainland United States are not "immigrants" in the literal sense of the term.
The majority of Puerto Rican immigration to the United States has occurred since the end of the Second World War. As of 1940, there were fewer than 70,000 Puerto Ricans living on the mainland. After the war, an abundance of employment opportunities to drew large numbers of Puerto Ricans to New York and other U.S. industrial centers. Today, there are more than 2.5 million people of Puerto Rican ancestry living in the United States, with almost 1 million of these residing in New York City.
Puerto Rican immigration has tended to be highly fluid, with many Puerto Ricans moving to the mainland and back to the island several times during the course of their lives.