Evidence is presented to show that density fluctuations that occur in van der Waals and polar liquids can be ascribed to voids approximately equal to a molecular volume. On the basis of a simple quasilattice model for the liquid a velocity autocorrelation function is derived and compared with the result of computer experiments on liquid argon. This comparison is made with the choice of three parameters: the mean time τc between ``hard'' collisions and the mean barrier heights for the creation of an interstitial molecule in the absence (ε¯0′) and presence (ε¯0) of a nearest-neighbor vacancy. A general expression for the self-diffusion coefficient of a liquid is derived in terms of τ c, ε¯0, and ε¯0′. Calculated pre-exponential factors (D∞) for self-diffusion are compared with experimental values for a variety of van der Waals and polar liquids. For simple liquids the agreement is good but for more complex liquids the observed values of D∞ are larger than the calculated values which are ascribed to an entropy of activation. The entropy of activation needed is approximately proportional to the energy of activation for diffusion. Barrier heights ε¯0 correlate well with the Lennard-Jones energy parameter for the free gaseous molecules. Volumes of activation for diffusion and relaxation are compared with experiment for argon, benzene, and chlorobenzene. An extension of the theory to solid-state diffusion is outlined.