  Observations of the gamma-ray sky reveal the most powerful sources and the
most violent events in the Universe. While at lower wavebands the observed
emission is generally dominated by thermal processes, the gamma-ray sky
provides us with a view on the non-thermal Universe. Here particles are
accelerated to extreme relativistic energies by mechanisms which are still
poorly understood, and nuclear reactions are synthesizing the basic
constituents of our world. Cosmic accelerators and cosmic explosions are the
major science themes that are addressed in the gamma-ray regime.
  With the INTEGRAL observatory, ESA has provided a unique tool to the
astronomical community revealing hundreds of sources, new classes of objects,
extraordinary views of antimatter annihilation in our Galaxy, and fingerprints
of recent nucleosynthesis processes. While INTEGRAL provides the global
overview over the soft gamma-ray sky, there is a growing need to perform
deeper, more focused investigations of gamma-ray sources. In soft X-rays a
comparable step was taken going from the Einstein and the EXOSAT satellites to
the Chandra and XMM/Newton observatories. Technological advances in the past
years in the domain of gamma-ray focusing using Laue diffraction and
multilayer-coated mirror techniques have paved the way towards a gamma-ray
mission, providing major improvements compared to past missions regarding
sensitivity and angular resolution. Such a future Gamma-Ray Imager will allow
to study particle acceleration processes and explosion physics in unprecedented
detail, providing essential clues on the innermost nature of the most violent
and most energetic processes in the Universe.
