Reliable prediction of spray penetration and spray break-up is required to achieve increases in fuel efficiency and reduction of emissions in diesel engines. Of particular interest is the early transient-flow regime. In the current work, diesel fuel spray development was studied using high-speed imaging of a high-pressure diesel common-rail fuel injector mounted in a spherical constant volume combustion chamber. The fuel injector nozzle had four holes aligned on a radial plane with diameters of 90, 110, 130, and 150 mu m. Fuel was injected into a room temperature T = 298 K (+/- 1.5%), nitrogen environment at chamber densities of 17.5, 24.2, and 32.7 kg/m(3) (+/- 3%) and for fuel-rail pressures of 1000, 1500, and 2000 bar (+/- 1.5%). High-speed images of the backlit fuel injection were captured at 100,000 frames per second. Image processing algorithms were used to determine fuel spray penetration distance and maximum penetration rate as a function of time. The early time history of the spray penetration was not sensitive to orifice size, orifice location or chamber density at the conditions studied. However, fuel injection pressure significantly affected the spray-tip penetration and time to spray break-up. The experimental results for maximum penetration rate and transition time were compared with various quasi-one-dimensional fuel-spray models. The experimental data indicated a power law relationship for the spray-tip penetration at early times in the spray development, which is consistent with recent recommendations. However, the experimental results for time to spray break up departed from the model predictions at most of the conditions studied, with the model significantly under-predicting the time for spray transition. Furthermore, the spray penetration data showed significant fluctuations in the spray geometry at early times. A fuel spray-tip tracking algorithm was developed and the results showed the maximum penetration distance did not occur along the spray center-line during the transient period of injection and the results quantified the angular location of the maximum penetration distance. These data provide valuable new insights into transient fuel spray behavior and will guide the development of the next generation of spray theory and models. (C) 2013 Elsevier Ltd. All rights reserved.