Clouds in the upper troposphere and lower stratosphere (UTLS) have important chemical and radiative effects. Observations of UTLS clouds are now pushing us to develop improved models with which to diagnose and forecast these chemical and radiative effects. This review provides a brief historical overview, describes some of the most important new observations of polar stratospheric and tropical tropopause clouds, and discusses likely impacts of climate change on the clouds. The review also outlines an approach to modelling the microphysics of the non-equilibrium solution aerosol of the kind that has been shown to be important in the polar stratosphere.