 The cluster ion spectrometry, CIS, experiment on the four cluster spacecraft measures the full three-dimensional ion distribution of major magnetospheric ions from thermal energies to about 40 keV slash E using two different instruments, a composition and distribution function analyzer, CIS1 slash codiff, and a hot ion analyzer, CIS2 slash HIA. The experiment has two different sensitivities to increase dynamic range, and first results show high-level performances and capabilities in various magnetospheric and solar wind magnetosphere interaction regions. This article was authored by H. Reem, C. Osten, J. M. Boscade, and others.