Resonance phenomena in collisions of atomic ions with electrons and photons

An overview is given on the physics of intermediate autoionizing states occurring in various observation channels of electron-ion and photon-ion interactions. Formation and decay processes of such resonances are numerous and involve intricate effects on the associated cross sections. Recent progress in the field has become possible by the combination of intense, high quality photon and electron beams with well prepared ionic targets. By using advanced ion source technology, storage ring capabilities and trapped ion techniques a wide spectrum of target species is accessible to studies of ionic resonances ranging from negative to highly charged positive ions and even to ions encapsulated within fullerene molecules.