After Einstein and Oppenheimer, Niels Bohr was the most influential physicist of the 20th century. He was the central figure in the development of quantum mechanics (contra Einstein with whom he clashed). Due to his Jewish ancestry he had to escape from his native Denmark during the Second World War and ended up working on the Manhattan project to build an Atomic bomb for America, a programme run by Robert Oppenheimer who idolised him. This first popular biography tells his remarkable life story, including what really happened in 1943 in Copenhagen when he famously challenged his former student Werner Heisenberg for working on the Nazi atomic bomb, and lucidly explains his revolutionary scientific ideas.