The theory for electron detachment in low-energy collisions of negative ions and atoms and experimental methods for studying this detachment are reviewed. Theory and experiment are compared, particularly with regard to the energy spectra of the emitted electrons. The relationship between electron detachment and nonequilibrium processes in plasmas is discussed, as is the relationship between detachment and the occupation of highly excited states of the colliding particles. The typical parameters of the process, the possibility of extracting them from theory and experiment, and the various theoretical methods and models for detachment and ionization are all discussed. The excitation and decay of approximately stationary states (autoionization and autodetachment states) of quasimolecules and also of isolated atoms and negative ions are discussed. Isotopic effects and the corresponding theory are discussed. Information on the behavior of unstable terms of the colliding atomic particles and of the widths of these terms cannot be obtained by other methods, so that this branch of the physics of atomic collisions may be singled out for separate discussion as the ``electron spectroscopy of unstable quasimolecular states.''