A brief review on the field of Solid State Ionics, including the diffusion pathway of mobile ions, crystal structure and materials, is presented. In the fluorite-structured ionic conductors such as ceria solid solution Ce_0.93Y_0.07O_1.96, bismuth oxide solid solution δ-Bi_1.4Yb_0.6O₃ and copper iodide CuI, a similar curved diffusion pathway along the <100> directions is observed. In the ionic and mixed conductors with the cubic ABO₃ perovskite-type structure such as lanthanum gallate and lanthanum cobaltite solid solutions, the mobile ions diffuse along a curved line keeping the interatomic distance between the B cation and O^2- anion to some degree. The structure and diffusion path of double-perovskite-type La_0.64Ti_0.92Nb_0.08O_2.99, K₂NiF₄-type (Pr_0.9La_0.1)₂(Ni_0.74Cu_0.21Ga_0.05)O_4+δ , and apatite-type La_9.69(Si_5.70Mg_0.30)O_26.24 are described. The diffusion paths of Li⁺ ions in La_0.62Li_0.16TiO₃ and Li_0.6FePO₄ are two- and one-dimensional, respectively.