The wave motion of fluids in finite containers, called slosh, is known to have adverse effects on the dynamics of aerospace vehicles and tanker trucks as well as on large storage tanks during earthquakes. This paper investigates slosh from an active feedback control perspective, and considers two possible active control methods for attenuating the response of the fluid to an external disturbance acceleration acting on the tank. The first method uses surface pressure control, whereas the second method uses a flap actuator on the surface of the fluid. In the first part of the paper we derive a state space model of slosh in a tank of rectangular cross section. This model is then used to design feedback controllers using LQG synthesis, and simulation results are presented to demonstrate closed-loop performance.