The stratosphere accumulates the bulk of meteoric and cometary material that impacts the Earth and provides a measure of that flux. Recent models and measurements of the effective age of stratospheric air demonstrate our ability to simulate this buildup. Current observations of H2O in the stratosphere and mesosphere are consistent with an extraterrestrial source of H2O, but limit its influx to less than 2 Tg/yr. Such a flux is consistent with standard estimates but is 100 times less than the small-comet hypothesis. ¿ 1998 American Geophysical Union