Propellant sloshing inside a partially filled tank onboard a spacecraft is a major disturbance for attitude and orbit control, causing performance degradations and in severe cases even an unstable spacecraft. Hence the impact of a sloshing propellant on the spacecraft’s structure and thus on the Attitude & Orbit Control System needs to be appropriately considered in spacecraft design and verification. Existing propellant slosh modeling approaches comprise mechanical analog models, which mainly consider linear sloshing motion only, and computational models, which lack the accurate and robust description of generic nonlinear sloshing motion in both high-gravity and low-gravity environments as for example high-velocity impacts on the tank wall or generic wetting effects including contact angle hysteresis. Additionally, the high computational cost of the currently existing computational models makes it impossible to use them directly in Attitude & Orbit Control System verification campaigns. The main objective of this thesis is to develop a computational propellant sloshing model able to reproduce both linear as well as nonlinear sloshing effects typically arising in spacecraft missions. The derived computational model is validated for high-gravity environments through hydrostatic and dynamic sloshing tests, comparing the simulation results to theory, experiments and other numerical models. A validation for low-gravity conditions is done by simulating surface tension as well as wetting effects and comparing the results to theory. Optimizing the computational propellant sloshing model as well as its software implementation for shorter simulation runtimes and better usability is the second objective of this work. The presented computational propellant sloshing model is capable of simulating problems using a low spatial resolution and thus a low simulation runtime, while still providing global system properties with sufficient accuracy. Additionally, the software implementation of the computational model uses a lightweight parallelization technique that is not adding too much overhead when using low spatial resolutions. The usability of the software is increased by providing a convenient way of pre-processing including automatic geometry creation as well as post-processing comprising data visualization, data analysis and export of simulation results. Also, all parameters used to setup a simulation have a real physical meaning, like e.g. dynamic viscosity or surface tension, hence no parameters specific to the chosen numerical method need to be configured. Finally, the third objective of the thesis at hand is the validation of the computational propellant sloshing model for Attitude & Orbit Control System verification simulations. This is achieved by developing a generic spacecraft Attitude & Orbit Control System simulator and coupling it to the derived propellant sloshing simulator. Two coupled simulations are performed covering applications of nonlinear propellant sloshing in high-gravity and zero-gravity conditions, showing that the developed computational propellant sloshing model is well suited to be used in Attitude & Orbit Control System verification campaigns.