An experimental and numerical study has been conducted of drop trajectories and breakup mechanisms for liquid drops injected into high-velocity gas flows with various chamber gas pressures at room temperature. In the experimental study, air-assisted liquid drop atomization processes were investigated using photographic techniques under well-controlled experimental conditions. A monodisperse stream of drops from a vibrating-orifice drop generator was injected into a transverse high-velocity gas stream. The back pressures and gas velocities were adjusted independently to control the drop Weber numbers. The Weber numbers used in the experiments were 72, 148, 270, and 532. High-magnification photographs and conventional spray-field photography revealed the microscopic breakup mechanisms and the parent drop trajectory in the high-velocity flow field, respectively. Drop sizes were measured using a phase/Doppler particle analyzer. The experimental results were used to test and assess spray models in the KIVA3 code. The breakup model considered Kelvin-Helmholtz (K-H) instability mechanisms to account for secondary drop breakup. The computations show good agreement with experimental results of parent drop trajectories and for the special drop size distributions which result from secondary breakup at high gas densities. At low gas densities, it is concluded that the use of the K-H model to predict drop breakup may not be justified, since the drops are observed to break up in the bag-breakup regime which is characterized by membrane stretching breakup.