  The international planetary science community met in London in January 2020,
united in the goal of realising the first dedicated robotic mission to the
distant Ice Giants, Uranus and Neptune, as the only major class of Solar System
planet yet to be comprehensively explored. Ice-Giant-sized worlds appear to be
a common outcome of the planet formation process, and pose unique and extreme
tests of our understanding of planetary origins, exotic water-rich planetary
interiors, dynamic seasonal atmospheres, complex magnetospheric configurations,
geologically-rich icy satellites (both natural and captured), and delicate
planetary rings. This article introduces a special issue of Philosophical
Transactions of the Royal Society A on Ice Giant System exploration at the
start of the 2020s. We review the scientific potential and existing mission
design concepts for an ambitious international partnership for exploring Uranus
and/or Neptune in the coming decades.
