The liquid in a tank on a navigating vehicle will slosh about and exert reaction forces on its enclosure. If the vehicle is a spacecraft, the liquid-free surface is not simply flat, and if the liquid and the solid masses are comparable in magnitude, the dynamics of the system are coupled, generally in all three dimensions. Qualitative features of the liquid motion are discussed, with emphasis on methods of analysis. The choice of a coordinate system attached to the tank introduces its motion as a field in the Navier-Stokes equations. Damping is identified as a key problem. Computational Fluid Dynamics is the method to analyse large motions of the liquid free surface. The instability from large liquid-solid mass ratio is discussed, with its cure. Some problems experienced by actual spacecraft are recalled. Introduced is Sloshsat F LEVO, a small spacecraft designed to investigate liquid dynamics and its effect on system motion.