  The recently adopted Ariel ESA mission will measure the atmospheric
composition of a large number of exoplanets. This information will then be used
to better constrain planetary bulk compositions. While the connection between
the composition of a planetary atmosphere and the bulk interior is still being
investigated, the combination of the atmospheric composition with the measured
mass and radius of exoplanets will push the field of exoplanet characterisation
to the next level, and provide new insights of the nature of planets in our
galaxy. In this white paper, we outline the ongoing activities of the interior
working group of the {\it Ariel} mission, and list the desirable theoretical
developments as well as the challenges in linking planetary atmospheres, bulk
composition and interior structure.
