This review focuses on the use of accelerator-based methods to investigate the interaction between negative ions and photons, electrons, heavy particles and external electric fields. The goal of negative ion physics is to better understand the role played by electron correlation in the structure and dynamics of many-electron systems. Negative ions are well suited for such studies since they exhibit an enhanced sensitivity to correlation due to the efficient screening of the nucleus by the atomic electrons. The structure of a negative ion is qualitatively different from that of an atom or positive ion. The difference can be traced to the nature of the force binding the outermost electron. In the case of a negative ion, the extra electron moves in a short-range potential arising from the induced dipole associated with the polarization of the atomic core. Typically, this potential supports only a single bound state. Negative ions are studied experimentally by detaching one or more electrons in a controlled manner. Excited states of negative ions are often found embedded in continua above the first detachment threshold. These transient states decay by autodetachment and their presence is manifested as a resonance structure in detachment cross sections.