This chapter provides an overview of the elementary physics and chemistry underlying nuclear energy, i.e., nuclear fission (“splitting” of nuclei) and nuclear fusion (joining them under high temperature and pressure). The chapter summarizes the state of the art on nuclear forces and the atom, radiation, and the necessary protection against adverse biological effects, and then explains physical principles of nuclear fission and nuclear fusion. The chapter also provides an in-depth description of reactor technology lines and specific reactor concepts, which are currently operating or are under development. For nuclear fission, we distinguish between the dominant light-water reactors and so-called “new” reactor concepts (SNRs), which have not yet diffused in the industry. For nuclear fusion, the main technology lines are magnetic and inertia confinement. The chapter also puts the developments into historical context.