Niels Bohr was a Noble Prize winning Danish physicist who did pioneering work in quantum theory and in contributing to the understanding of atomic structure. Born to a highly influential and well educated famil, he is regarded as one of the most dominant physicists of the 20th century. After earning his doctoral degree in physics, he conducted an intensive research along with Ernest Rutherford on the atomic structures. He formulated the first successful explanation of some major lines of the hydrogen spectrum and his theory of the atom became the foundation of modern atomic physics. His remarkable contribution to the understanding of the atomic structure and quantum mechanics earned him the Nobel Prize in Physics. Among other things, he also proposed the complementarity principle, which states that objects may have a dual nature, similar to that of an electron which behaves both as a particle and a wave, but we can only experience one aspect at a time. During World War II, he escaped arrest by German police and eventually made it to the United States where he acted as a prominent part of the team of physicists working on the Manhattan Project. He was also a noted humanitarian and after the war, he spent the rest of his life advocating the peaceful use of nuclear energy.