This publication contains five articles of Niels Bohr, which were written in five languages–English, German and Danish. Niels Bohr had an outstanding role in the history of quantum mechanics. His investments in science was in philosophy as well as in physics because the problems that put forward the experiments in late 19th and early 20th century could not be solved without revising the most stable philosophical grounds for the previous physics. Bohr's five articles presented here, reflect the complex and difficult process of changing the philosophical and physical ways of thinking that lead to the creation of quantum mechanics, a new and still active developing branch of science.