Collisions between ions belong to the elementary processes occuring in all types of plasmas, in astrophysical objects as well as in laboratory discharges. In recent years, the impetus for accurate cross section data for such collisions has become greater due to research in thermonuclear fusion using either magnetic or inertial confinement.

In this paper experimental data, measured by the crossed-beams technique, for charge transfer and ionization in ion-ion collisions at keV energies are reviewed. In addition to collisions between two singly-charged ions, either positively or negatively charged, also data for collisions between multiply-charged ions have become available in recent years due to the advent of ECR ion sources. Of special interest are the one-electron and quasi-one electron collision systems, which provide an ideal testing ground for different theoretical models.

Recent experimental progress includes first ever measurements of angular differential cross sections and state-selective electron capture in ion-ion collisions. Also, charge transfer between two fullerene ions could be studied for the first time.