To be eligible for naturalization in the United States, applicants must meet several legal standards. They must enter the country lawfully, obtain legal permanent resident status, and reside in the U.S. for five continuous years, or three years if married to a U.S. citizen. Additionally, they must be physically present in the country for at least half of the residency period, be at least 18 years old, and have the ability to understand, speak, read, and write basic English. Good moral character and an attachment to the constitutional principles of the U.S. are also required, along with taking an oath of allegiance to the United States.