Air may cross the tropical tropopause either by gradual ascent or in localized episodes associated with convection. While observations demonstrate that water vapor mixing ratios of air entering the tropical stratosphere are consistent with the mean tropical tropopause temperature, they do not resolve key mechanistic questions, such as the relative contribution of gradual or episodic transport, or the role of thin cirrus. As Moyer et al. <1996> clearly argue, observations of the isotopic content of water entering the tropical stratosphere can provide a strong constraint on models of water vapor transport across the tropopause. For example, stratospheric HDO is too abundant to be compatible with the assumption that all moisture enters the stratosphere as vapor during convection. Analysis of recent H218O observations shows that kinetic effects cannot explain the HDO excess. Lofting and evaporation of cloud ice can explain the observed stratospheric water vapor content and its isotopic composition, but the relative importance of gradual or episodic transport remains unresolved. ¿ 2000 American Geophysical Union