  The era of outer planet orbiters (Galileo, Juno and Cassini) is advancing our
understanding of how the radiation belts of Jupiter and Saturn are structured,
form and evolve well beyond what had been possible during the age of flyby
missions and ground-based observations. The nearly two decades-long datasets of
these missions, in the context of detailed and long-term observations of
Earth's radiation belts, highlight which of the processes that accelerate
particles to relativistic kinetic energies and limit their flux intensity can
be considered more universal, and thus key for most extraterrestrial
magnetospheres, and which reflect the unique aspects of each planet and its
magnetospheric system. In this chapter we focus on the in-situ radiation belt
observations in the context of theory, simulations and relevant measurements by
Earth-based observatories. We describe both the average state and the time
variations of Jupiter's and Saturn's radiation belts and associate them with
specific physical processes.
