  We review the current understanding of the upper atmospheres of Uranus and
Neptune, and explore the upcoming opportunities available to study these
exciting planets. The ice giants are the least understood planets in the solar
system, having been only visited by a single spacecraft, in 1986 and 1989,
respectively. The upper atmosphere plays a critical role in connecting the
atmosphere to the forces and processes contained within the magnetic field. For
example, auroral current systems can drive charged particles into the
atmosphere, heating it by way of Joule heating. Ground-based observations of
H$_3^+$ provides a powerful remote diagnostic of the physical properties and
processes that occur within the upper atmosphere, and a rich data set exists
for Uranus. These observations span almost three decades and have revealed that
the upper atmosphere has continuously cooled between 1992 and 2018 at about 8
K/year, from $\sim$750 K to $\sim$500 K. The reason for this trend remain
unclear, but could be related to seasonally driven changes in the Joule heating
rates due to the tilted and offset magnetic field, or could be related to
changing vertical distributions of hydrocarbons. H$_3^+$ has not yet been
detected at Neptune, but this discovery provides low-hanging fruit for upcoming
facilities such as the James Webb Space Telescope (JWST) and the next
generation of 30 metre telescopes. Detecting H$_3^+$ at Neptune would enable
the characterisation of its upper atmosphere for the first time since 1989. To
fully understand the ice giants we need dedicated orbital missions, in the same
way the Cassini spacecraft explored Saturn. Only by combining in-situ
observations of the magnetic field with in-orbit remote sensing can we get the
complete picture of how energy moves between the atmosphere and the magnetic
field.
