Possible diffusion paths in lithium niobate are discussed. The preferred path for diffusion is strongly influenced by the distribution of ions between the lithium and niobium sites; site swapping is important both for creating non-stoichiometric material and for impurity incorporation. Various literature reports of diffusion rates are surveyed for comparison with the possible diffusion mechanisms. Titanium (which is thought to substitute primarily for niobium) appears to diffuse more rapidly on the lithium sublattice in lithium-deficient samples.