The book addresses the topic of hydrogen as an important secondary energy carrier for renewable primary energies. It provides an overview of the state of the art and the development and market potential in the fields of energy technology, mobile, stationary, and portable applications, uninterruptible power supply, and the chemical industry. The authors are scientists and experienced practitioners. It is particularly aimed at engineers, chemists, business economists, as well as students and researchers. Three years after the release of the first edition, the second edition is now available, in which all chapters have been updated according to the state of development. Additionally, two chapters have been added: hydrogen storage in salt caverns and hydrogen - a key element of Power-to-X, whose content has gained significance in the meantime.