Underwater autonomous vehicles are frequently used for deep-water ocean applications such as surveying and cable-laying, where accurate control of vehicle depth and attitude is needed. The water level in the on-board ballast tanks are typically manually set for neutral buoyancy before each mission, while the vehicle is on the surface. The resulting weight of the water level is not normally adjusted while the unmanned vehicle is in operation to control vehicle depth and orientation. As a result, vehicle trajectory and orientation is exclusively controlled using the vehicle’s control surfaces during a mission. The challenges with controlling the depth and trim of an underwater vehicle include nonlinear hydrodynamic forces as well as relatively slow response times and inherent time delays (latencies) associated with water tank level changes and valve adjustments. To meet these challenges, this thesis proposes two unique variable ballast system control approaches. The proposed control approaches may be suitable for large autonomous underwater vehicles with both small (volume = 0.027 m3, each) and large (volume = 0.216 m3, each) ballast tanks. The first proposed variable ballast system controller uses the current parameters of the ballast tanks to determine the appropriate action to be implemented. This controller was designed change the weight of the AUV to help control vehicle parameters such as depth and vertical (inertial) velocity. The second proposed variable ballast controller attempts to shift the center of gravity x_G along the body-fixed x-(longitudinal) axis by changing the weight in the ballast tanks. By shifting the center of gravity, the controller attempts to reduce depth and pitch angle error while regaining control authority to the bowplane and sternplane deflection fins. The ballasting system consists of two water tanks positioned aft and forward of amidships. The ballast tanks are then automatically filled or emptied of ocean water as desired. Setpoint depth control and x_G shifting numerical simulations have been carried out on a two-dimensional underwater vehicle simulator to test and compare the performance of the proposed ballast and deflection control systems. The simulation results show that, for the assumptions and conditions tested, the proposed controllers are versatile and capable of achieving a setpoint depth and pitch angle with minimal error by effectively utilizing the ballast tanks and deflection fins. As a result, the work presented in this thesis helps increase the autonomy of large AUVs on long duration missions.