A study was performed of the distortion and breakup mechanisms of liquid drops injected into a transverse high velocity air jet at room temperature and atmospheric pressure. The investigation included the use of ultra-high magnification, short-exposure photography to study the three drop breakup regimes previously referred to as the bag breakup regime, the shear or boundary-layer stripping breakup regime, and the ’catastrophic’ breakup regime. in the experiments the initial diameters of the injected diesel fuel drops were 69, 121 and 198 mu m, and the transverse air jet velocity was varied from 68 to 331 m/s. The experimental conditions correspond to drop initial Weber numbers of 56, 260 and 463 for the three breakup regimes. The drop Reynolds numbers (based on gas properties) ranged from 509 to 2488. It was found that the drop breakup process occurs in two stages. During the first stage, under the action of aerodynamic pressure, the drop distorts from its undisturbed spherical shape and becomes flattened, or disk shaped, normal to the air flow direction. This feature exists in all three drop breakup regimes. A dynamic drag model that is a modified version of the DDB (Dynamic Drag and Breakup) model and accounts for the increase of both the drop’s frontal area and its drag coefficient as a function of its distortion was used to analyze the drop trajectory and its distortion during the first stage of the drop breakup process. During the second stage of the drop breakup process, the three drop breakup regimes display different breakup features. In the bag breakup regime the appearance and growth of holes on the bag sheet blown out of the center of the flattened drop is the dominant reason for the breakup; in the so-called shear or boundary-layer stripping breakup regime the results indicate that bending of the flattened drop’s edge under the action of aerodynamic pressure, followed by production of folds on the bent sheet results in production of ligaments aligned in the direction of the air flow; and in the ’catastrophic’ breakup regime the growth of capillary waves on the flattened drop surfaces, combined with the bending and folding of the sheet edge makes the breakup process demonstrate ’catastrophic’ breakup characteristics. In addition, the experimental results confirm that for drops with different sizes, the same breakup regimes appear when the Weber number is held constant, and the Reynolds number does not play a dominant role. These results thus cast considerable doubt on the validity of the widely used ’shear’ or ’boundary-layer stripping’ drop breakup theories in which viscous effects would be important.