The break up of a liquid jet in cross flow has applications in fuel atomization processes. The break up of a water jet in a high speed cross flow was studied with momentum ratios ranging between 10 and 172. High-speed camera images showed break up characteristics ranging from bag break up (break up due to large ligaments or "bags" of fluid sheared off the liquid jet) to multimode break up (break up in which large ligaments and small drops are present), with large disturbances developing on the jet boundary. The disturbance wavelengths and break up locations were measured and compared, and the agreement was very good. It was also observed that as the cross flow velocity increased, the jet boundary spread linearly outward in the spanwise direction. Particle Image Velocimetry (PIV) results showed that the cross flow did not follow the jet boundary, but passed around the jet, similar to the flow around a bluff body. This implies that the Rayleigh-Taylor instability cannot be a dominant mechanism for the jet break-up. Spanwise PIV results indicate the presence of a high shear region along the sides of the jet, which might serve as the primary cause of jet break-up.