Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron (changing a nuclear proton to a neutron) and simultaneously emits a neutrino. Various photon emissions follow, in order to allow the energy of the atom to fall to the ground state of the new nuclide.

Electron capture is the primary decay mode for isotopes with a relative superabundance of protons in the nucleus, but with insufficient energy difference between the isotope and its prospective daughter (with one less positive charge) for the nuclide to decay by emitting a positron. Electron capture also exists as a viable decay mode for radioactive isotopes with sufficient energy to decay by positron emission, where it competes with positron emission. It is sometimes called inverse beta decay, though this term can also refer to the capture of a neutrino through a similar process.

If the energy difference between the parent atom and the daughter atom is less than 1.022 MeV, positron emission is forbidden because not enough decay energy is available to allow it, and thus electron capture is the sole decay mode. For example, rubidium-83 (37 protons, 46 neutrons) will decay to krypton-83 (36 protons, 47 neutrons) solely by electron capture (the energy difference, or decay energy, is about 0.9 MeV).