An atomic or molecular system having an excess of negative charge. Negative ions, also called anions, are formed in attachment processes in which an additional electron is captured by an atom, molecule, or cluster. They can also be formed when a molecule or cluster dissociates. Doubly charged negative ions, also called dianions, have also been observed in the case of molecules and clusters. Here, two additional electrons have become attached to the neutral systems. Negative ions are destroyed in a controlled manner in detachment processes and, in the case of molecular ions or clusters, dissociation processes, when the ion interacts with photons, electrons, heavy particles, or external fields. Experimental studies of negative ions involve measurements of cross sections for detachment and dissociation.