The following study is a comparison of the National Center for Atmospheric Research (NCAR) isentropic trajectory model and the Air Resources Laboratories Atmospheric Transport and Dispersion (ARL/ATAD) variable height trajectory model. Back-trajectories from each model were computed within the boundary layer and at two levels above the boundary layer over periods in July and December of 1977 and February of 1978. All trajectories terminated in Charlottesville, Virginia. Assessment of the models was achieved through the study of potential vorticity measurements computed along NCAR trajectories, wind shear values computed along the ARL/ATAD trajectories, as well as through consideration of synoptic patterns available during the case study periods. A root mean square (r.m.s.) analysis was performed to quantify model differences. Results of this study show that in the absence of frontal movement the ARL/ATAD model is a better choice within the boundary layer, but only because the model is less expensive to run. NCAR trajectories are probably more realistic in the vicinity of fronts or other large sources of vertical movement. Above the boundary layer, both models produce similar trajectories when the atmosphere is barotropic; NCAR trajectories appear more accurate in baroclinic atmospheres because of better treatment of vertical motion, r.m.s. studies show that NCAR and ARL/ATAD trajectories differ more during winter than summer, especially after 48 h of trajectory calculation r.m.s. trajectory differences remain similar for different levels for a given season and period of time.