Understanding the process of primary breakup of liquids of air-assisted atomization systems is of major importance for the optimization of spray nozzles. Experimental investigations face various limitations. Conventional numerical methods for the simulation of the process are subject of various shortcomings. In the present paper the “Smoothed Particle Hydrodynamics” (SPH)-method and its advantages over conventional methods are presented. The suitability of the method is demonstrated by analyzing two different air-assisted atomization systems. First, a methodology for the investigation of the two-phase flow in fuel spray nozzles for aero-engine combustors is introduced and predictions of the flow in a typical nozzle geometry are presented. Second, numerical simulations of the atomization of a viscous slurry, which is used in gasifiers for biofuel production, are presented and compared to experimental results. Highly valuable results are retrieved, which will improve the fundamental understanding of primary breakup and will enable to optimize nozzle geometries.