Recent observations from aircraft, balloons, and satellites, along with advances in modeling techniques and improved meteorological analyses, are providing new insights into the physical processes controlling clouds and water vapor concentration in the tropical tropopause and lowermost stratosphere. In particular, this session will focus on model and observational investigations addressing the impacts of rapid transport by deep convection, slow diabatic vertical motions, wave-driven temperature oscillations, and cloud microphysics using analyses from recent aircraft and balloon in situ measurements and long-term satellite measurements.