There has been tremendous change over the last few decades in the operating conditions of diesel fuel injection systems and engines, and in the diagnostic tools and numerical models available to evaluate them, Improvements in the diagnostic techniques coinciding with changes in diesel injector technology brought about an entirely different view of the breakup of liquid in current diesel sprays. A detailed examination of the history and current understanding of the structure of the dense core region in transient diesel spray ir presented. Diagnostic methods are reviewed, and the appropriate uses are discussed. Of the techniques currently available, tomography is the most appropriate for determining the structure of the dense core region. Conductivity is not recommended. Line-of-sight techniques are recommended only for studying the periphery of the spray. Due to it＇s greater contrast, high-intensity Mie scattering is preferred over line-of-sight methods for liquid spray penetration distance measurements. Advances in phase-Doppler interferometry are required to provide drop size and velocity measurements in the near-nozzel region. A review of the spray structure and breakup mechanisms is presented. The structure of the spray has been shown to be completely atomized at or near the nozzle tip, with nozzle cavitation and turbulence instabilities as the dominant breakup mechanisms. Buckling may be responsible of breakup during the very early phase of injection. Aerodynamics shear may cause some secondary atomization, but its role in breakup is far less significant than previously thought. Cavitation affects jet breakup through the bursting and collapsing vapour cavities, thus contributing to the disintegration of liquid, resulting in a mixture of bubbles and liquid occupying most of the cross-sectional area, and through increasing the turbulence intensity, thus contributing to the instability of the liquid jet. The turbulence instability, along with pressure fluctuations in the nozzle, cause variation in the exit velocity of the droplets, resulting in temporal and spatial clustering of the droplets in the plume. The results of recent research on the liquid spray penetration distance and drop size have been summarized. For liquid spray penetration distance, the orifice diameter id the dominant injection parameter, and ambient density is the dominant engine parameter, although ambient temperature is also significant, Fuel properties have been shown to have an effect on the liquid spray penetration distance, but further research is required to draw significant conclusions. For drop size, injection pressure and orifice diameter are the known dominant parameters. The evidence of complex atomization of diesel sprays near the nozzle has come from a cavity of sources, including tomographic imaging of the internal structure, microphotography of the near-nozzel region, diffraction droplet sizes that are greater on the periphery than the centerline, infrared multiwavelength extinction droplet sizing,a nd internal flow studies.