  Today, we know ~4330 exoplanets orbiting their host stars in ~3200 planetary
systems. The diversity of these exoplanets is large, and none of the known
exoplanets is a twin to any of the solar system planets, nor is any of the
known extrasolar planetary systems a twin of the solar system. Such diversity
on many scales and structural levels requires fundamental theoretical
approaches. Large efforts are underway to develop individual aspects of
exoplanet sciences, like exoplanet atmospheres, cloud formation, disk
chemistry, planet system dynamics, mantle convection, mass loss of planetary
atmospheres.
  The following challenges need to be addressed in tandem with observational
efforts. They provide the opportunity to progress our understanding of
exoplanets and their atmospheres by exploring our models as virtual
laboratories to fill gaps in observational data from different instruments and
missions, and taken at different instances of times: Challenge a) Building
complex models based on theoretical rigour that aim to understand the
interactions of atmospheric processes, to treat cloud formation and its
feedback onto the gas-phase chemistry and the energy budget of the planetary
atmosphere moving away from solar-system inspired parameterisations. Challenge
b) Enabling cloud modelling based on fundamental physio-chemical insights in
order to be applicable to the large and unexplored chemical, radiative and
thermodynamical parameter range of exoplanets in the universe. Challenge b)
will be explored in this chapter of the book ExoFrontiers.
