Albert Einstein was a German-born theoretical physicist and philosopher. He developed the theory of relativity, one of the pillars of modern physics. He is regarded as one of the most influential scientists of the 20th century. Einstein received the Nobel Prize in Physics in 1921 for his services to theoretical physics and especially for his discovery of the law of the photoelectric effect. He immigrated to the United States in 1933 and became an American citizen in 1940. Einstein died at the age of 76 in Princeton, New Jersey.