Hans Bethe introduces quantum theory as "the most important discovery of the twentieth century" and shows that quantum theory gave us "understanding and technology." He cites computers as a dramatic realization of applied quantum physics. Professor Bethe begins his personal recollections of the development of quantum theory in Lecture 1. While not following the historical chronology rigidly, he covers the period of the "old quantum theory," from Planck's first idea in 1900 to work on the Bohr atomic model in 1914.