The present paper is a review of our recent works on the crystal structures, structural disorders and diffusion paths of mobile ions in various bulk ionic and mixed conductors, which was presented in the Keynote Lecture of SSI-16 international conference. The reported studies were conducted based on the nuclear/electron density distributions obtained by a combined technique including Rietveld refinement, the maximum entropy method (MEM) and the MEM-based pattern fitting (MPF) of the neutron/synchrotron powder diffiraction data measured at high temperatures. Diffusion paths along the < 100 > directions are observed in various ionic conductors with fluorite-type structures such as delta-Bi1.4Yb0.6O3, Ce0.93Y0.070O1.96, Y0.785Ta0.215O1.715 and alpha-Cul. The diffusion paths of oxide ions in the cubic perovskite-type (La0.8Sr0.2)(Ga-0.8 Mg0.15Co0.05)O3-delta exhibit an arc shape away from the (Ga0.8Mg0.15Co0.05) cation, forming a three-dimensional network of curved diffusion paths. Double perovskite-type La-0.64(Ti0.92Nb0.08)O-2.99 shows a similar diffusion path near the (004) planes, forming a two-dimensional network of curved diffusion paths. The curved feature of the diffusion paths is observed in various oxide-ion conductors Such as delta-Bi1.4Yb0.6O3, (La0.8Sr0.2)(Ga0.8Mg0.15Co0.05)O3-delta and La-0.64(Ti0.92Nb0.08)O-2.99, as well as in the copper-cation conductor alpha-Cul and lithium-cation conductor La0.62Li0.16TiO3. (C) 2008 Elsevier B.V. All fights reserved.