  Spectral retrieval has long been a powerful tool for interpreting planetary
remote sensing observations. Flexible, parameterised, agnostic models are
coupled with inversion algorithms in order to infer atmospheric properties
directly from observations, with minimal reliance on physical assumptions. This
approach, originally developed for application to Earth satellite data and
subsequently observations of other Solar System planets, has been recently and
successfully applied to transit, eclipse and phase curve spectra of transiting
exoplanets. In this review, we present the current state-of-the-art in terms of
our ability to accurately retrieve information about atmospheric chemistry,
temperature, clouds and spatial variability; we discuss the limitations of
this, both in the available data and modelling strategies used; and we
recommend approaches for future improvement.
