The analytical ultracentrifuge is a high speed centrifuge with an optical system allowing observation of the concentration of macromolecules as a function of radius and time. In sedimentation velocity experiments,relatively high speeds are used so that a boundary is formed between the solution of sedimenting macromolecule and the buffer in which it is dissolved. Analysis of the rate boundary movement and evolution of its shape can yield information about the molar masses of species present as well as stoichiometries and equilibrium constants for their interactions. This overview discusses crucial issues pertaining to sedimentation velocity experiments, including non-interacting and interacting systems, ideality and non-ideality, and reversible versus kinetically limited equilibrium scenarios.