Atomic nuclei consist of neutrons and protons bound by the strong force. Guided by the quantum-mechanical nature of the nuclear many-body system—driven, for example, by the Pauli exclusion principle and the concept of isospin—the attraction between a neutron and a proton is stronger than the attraction between two protons or between two neutrons. As a consequence, most stable, light nuclei are composed of equal numbers of protons and neutrons. Starting at about calcium (20 protons), the Coulomb repulsion between the positively charged protons reduces the binding of the nuclear system so that the most stable isotopes of an element have more neutrons than protons. Of the 3000 nuclei that have been produced in laboratories, only about 300 are stable. The other nuclei are short-lived and prone to decay until a stable nucleus is reached. The study of these so-called exotic nuclei that often exist only for fractions of a second has proven crucial for the understanding of the nuclear force and continues to provide important input for nuclear astrophysics in the quest to explain the isotopic composition of the universe.