Merged beams of electrons and accelerated highly charged ions have been employed to study resonant and non-resonant excitation phenomena in electron-ion collisions. In particular, dielectronic capture processes were observed with high collision-energy resolution both in the channels of net ionization and recombination. Ionized and recombined product ions were simultaneously detected downstream from the electron cooler in the Heidelberg test storage ring TSR with stored Li-like Si 11+, Cl 14+ ions as well as Na-like Cl 6+, Fe 15+ and Se 23+ ions. Absolute cross sections and rates for dielectronic and radiative recombination and for single ionization have been measured for electron-ion center-of-mass energies Ecm ranging from 0 eV up to several keV. In separate experiments at the GSI accelerator facilities absolute rates for recombination of Au 76+ and U 28+ were determined. Detailed spectroscopic information on the structure and stability of multiply excited states of highly charged ions were obtained.