  A description in terms of transition rates among cells is used to analyze
self-diffusion of hard spheres in the fluid phase. Cell size is assumed much
larger than the mean free path. Transition state theory is used to obtain an
equation that matches numerical results previously obtained by other authors.
Two regimes are identified. For small packing fraction $\xi$, diffusion is
limited by free volume; and, for large $\xi$, diffusion is limited by velocity
autocorrelation. The expressions obtained in each regime do not require
adjustable parameters.
