Atomization is a process ubiquitous in both nature and industry where a liquid flow is fragmented into a spray of small droplets. Despite having been studied extensively for almost a century, sprays remain poorly understood. The existing models are not based on a realistic understanding of the mechanisms of breakup and thus fail to provide good prediction of the spray behaviour across a wide range of operating conditions, in particular the size distribution of the spray droplets.In this thesis, the aerodynamic atomization of liquid drops and jets is studied experimentally and theoretically, with a focus on developing realistic models for the many sub-processes that occur during the breakup. It is shown that the initial deformation rate of a liquid drop exposed to a high speed gas flow governs its breakup morphology as well as the diameter of the ligaments produced from its deformation. Following the formation of these ligaments, a plurality of mechanisms are shown to occur throughout the fragmentation, which result in the distribution of droplet sizes. Analytical models are derived for each of the sub-processes of the breakup and compared with measurements of intermediate stages of droplet breakup, ultimately resulting in a prediction of the droplet size distribution from aerodynamic droplet breakup. Notably, the model presented in this thesis is unique in that it provides a complete description of the breakup process as well as a prediction of the resulting size distribution. Finally, the framework of the droplet breakup model is leveraged to model the atomization of a coaxial, twin-fluid spray, giving a deterministic, analytical prediction of the droplet size distribution resulting from twin-fluid atomization. The models developed in this work were made available in a Python implementation that we refer to as the Aerodynamic Droplet and Atomization Model (ADAM).