  Since the discovery of the first extrasolar planet more than twenty years
ago, we have discovered nearly four thousand planets orbiting stars other than
the Sun. Current observational instruments (on board the Hubble Space
Telescope, Spitzer, and on ground-based facilities) have allowed the scientific
community to obtain important information on the physical and chemical
properties of these planets. However, for a more in-depth characterisation of
these worlds, more powerful telescopes are needed. Thanks to the high
sensitivity of their instruments, the next generation of space observatories
(e.g. JWST, ARIEL) will provide observations of unprecedented quality, allowing
us to extract far more information than what was previously possible. Such high
quality observations will provide constraints on theoretical models of
exoplanet atmospheres and lead to a greater understanding of their physics and
chemistry. Important modelling efforts have been carried out during the past
few years, showing that numerous parameters and processes (such as the
elemental abundances, temperature, mixing, etc.) are likely to affect the
atmospheric composition of exoplanets and subsequently the observable spectra.
In this manuscript, we review the different parameters that can influence the
molecular composition of exoplanet atmospheres. We show that the high-precision
of ARIEL observations will improve our view and characterisation of exoplanet
atmospheres. We also consider future developments that are necessary to improve
atmospheric models, driven by the need to interpret the available observations.
