We present an analysis of the mechanisms controlling stratospheric water vapor based on in situ profiles made at 37.4¿ N and at altitudes up to 20 km. The stratosphere can be conveniently divided into two air masses: the overworld (potential temperature &thgr;>380 K) and the lowermost stratosphere (&thgr;<380). Our data support the canonical theory that air primarily enters the overworld by passing through the tropical tropopause. The low water vapor mixing ratios in the overworld, a few parts per million by volume (ppmv), are determined by the low temperatures encountered at the tropical tropopause, as well as oxidation of methane and molecular hydrogen. Air enters the lowermost stratosphere both by diabatically descending from the overworld across the 380-K potential temperature surface and by passing through the extratropical tropopause. Air parcels crossing the extratropical tropopause experience higher temperatures than air crossing the tropical tropopause, allowing higher water vapor in the lowermost stratosphere (tens of ppmv) than in the overworld. Our data are consistent with the pathway for air crossing the extratropical tropopause being isentropic advection from lower latitudes, although we cannot exclude contributions from other paths. ¿ American Geophysical Union 1995