Nuclear fission: from fundamentals to frontiers

Nuclear fission, the division of a heavy atomic nucleus into lighter fragments, is a process of fundamental importance to nuclear physics, energy production, and the origin of the elements. This chapter, part of the Encyclopedia of Nuclear Physics, provides a pedagogical introduction to a broad array of topics in fission physics, progressing from the macroscopic liquid-drop model and microscopic shell corrections through fission dynamics, observables, modern theoretical frameworks, and experimental techniques ranging from traditional double-energy methods to inverse kinematics at radioactive-ion-beam facilities. The roles of fission in astrophysical nucleosynthesis, nuclear energy, medical isotope production, and national security are discussed, and open questions including the predictive power of microscopic theory and the fission properties of neutron-rich nuclei relevant to heavy element nucleosynthesis are highlighted.