Charlize Theron has become a U.S. citizen.

The South African-born actress studied hard to pass the citizenship examination and insists becoming an American is something she has dreamed of for years.

Charlize said: "I've always wanted to be one, they just didn't want to take me. You're stuck with me now!"

"It's quite a process - you have to work hard, you have to study up. You have to know your stuff - you've got to know your senators, how the government works."

"They give you a test. It is what you should know and I think it is good that they do it. If you live here you should know your stuff. I did incredibly well, I studied up and I was right there with my answers before he had even finished the questions."

The 32-year-old 'Monster' star, who has been living in the U.S. for over a decade, also revealed her interview went smoothly until they asked her to write.

She added to talk show host David Letterman: "It was all going fine until the interviewer said, 'Take this piece of paper and write, 'It's a sunny day.' I was like, wait a second, this isn't in the book - I didn't study for that! It was one of those really weird feelings where you completely doubt yourself. I forgot how to speak English, I was thinking does sunny have a double n?"

"I guess they want to make sure that you can write and it turns out I can!"