Forward and backward three-dimensional stratospheric polar vortex trajectory computations between fall and spring for each of the years 1992--1993 through 1999--2000 in the Northern Hemisphere and the years 1992--1999 in the Southern Hemisphere have been carried out. We find that the forward and backward trajectories give very different pictures of polar vortex descent. The backward trajectories show a complex distribution of parcels in which one population originates in the upper stratosphere and mesosphere and experiences considerable descent in the polar regions, while the remaining parcels originate at lower altitudes of the middle and lower stratosphere and are mixed into the polar regions during vortex formation. The forward trajectory calculations do not show this second population since the parcels in the forward calculation originate within the protovortex. The amount of descent experienced by the first population shows little variability from year to year, while the computed descent and mixing of the remaining parcels show considerable interannual variability due to the varying polar meteorology. Spring methane values reconstructed from Halogen Occultation Experiment (HALOE) fall data and the back trajectories compare fairly well with typical HALOE spring values. These results imply that using a comparison between prewinter and postwinter tracer profiles to estimate the amount of descent over the fall-to-spring time period might result in unrealistically large values of descent, since the fall profiles are not representative of the same air as the spring profiles. Additional back trajectory calculations of descent in the lower stratosphere over 3-month time periods from spring to midwinter show that very little mixing occurs at these times and altitudes in the Northern Hemisphere during winters when the vortex is undisturbed, and in the Southern Hemisphere for all years studied. Thus, for this shorter time period, tracking constant mixing ratios of tracers within the vortex should give reasonable estimates of descent amounts, except for years when there are meteorological disturbances in the Northern Hemisphere. ¿ 2001 American Geophysical Union