Abstract:The ionic charge of energetic particles as observed in planetary environments and in interplanetary space provides crucial information on their source, is an important parameter for the diagnostic of the plasma conditions at the source region and provides fundamental information about the acceleration and propagation processes in planetary magnetospheres, at the Sun and in interplanetary space. In Earth’s magnetosphere, for example, the measurement of O+ and He+ is crucial for the determination of the ionosphere as the source of these particles. In Interplanetary Space the determination of the ionic charge allows to distinguish between twe different sources: the solar wind with heavy ion charge states compatible with coronal temperatures of ~1-3 MK, and interstellar neutrals, singly ionized in the inner heliosphere (pickup ions). In this paper we provide an overview of the experimental techniques, and discuss how ionic charge measurements help to better understand the two types of solar energetic particles (SEP) with striking differences of elemental (Fe/O), isotopic (3He/4He) and ionic charge (Fe) composition. The measurements of the mean ionic charge of SEPs over the large energy range of ~0.1 to 70 MeV/nuc allows, for example, to distinguish between acceleration in the high corona and interplanetary space and acceleration at low coronal altitudes where charge stripping effects are important. These charge stripping effects result in a specific signature of the energy dependence of the mean ionic charge and high charge states of Fe ions (~20) at energies above 10 MeV/nucleon, as observed in impulsive SEP events.