The composition of the upper troposphere and stratosphere plays a key role in the climate system. Our understanding of the interactions between dynamics, chemistry and climate in this region is rapidly advancing thanks to both observational and modelling studies. In this session, we invite presentations on dynamical, transport and chemical processes determining the variability and long-term trends in the composition of the UTLS, and related effects on radiation and dynamics. We particularly encourage contributions introducing recent observations (both in situ and remote sensing) as well as models of various complexity, ranging from comprehensive chemistry climate models to idealized, conceptual models, and theoretical studies. This year, a special focus will be laid on processes in the coupling region of the upper troposphere and lower stratosphere (UTLS), and we especially welcome related contributions from joint research projects (e.g. TPChange).