In this review, physical properties of the silver halide crystals that would be closely related to high efficiency of the latent image formation were discussed from the general point of view and then discussions were concentrated on the ionic process that was indispensable to the latent image formation. Ionic conductivity of the silver halide microcrystals dispersed in gelatin was measured by the dielectric loss method based on the Maxwell-Wagner-Sillars theory. It was shown that the ionic conductivity of the microcrystals was larger by several orders of magnitude than that of the large pure crystals, being proportional to the surf ace-to-volume ratio of the microcrystals and that the incorporation of divalent cadmium ions into the emulsions grains markedly decreased ionic conductivity.These results indicated that interstitial silver ions were the predominant charge carriers in ionic conduction, and that surface sites were responsible for the formation of the positive space-charge layer resulting in the existence of interstitial silver ions with high concentration in the microcrystals. Since the ionic carries were controlled by the surface process, the ionic conductivity was influenced by many emulsion variables, such as the grain size, surface structure of the grains and adsorption of the photographic addenta (for example, antifoggant, stabilizer and sensitizing dye). Effects of sulf ur and reduction sensitization and emulsion p Ag on the ionic conductivity were also examined in relation to the photographic effects. Distribution of Frenkel defects in a grain was studied theoretically and was examined experimentally by using iridium-doped AgBr grains. Further discussions were made on the effects of the space-charge layer upon the electronic process of the latent image formation. An attempt was made to obtain the relation between photographic sensitivity and the ionic conductivity of the emulsion grains. The emulsions containing the grains with low ionic conductivity showed high-intensity reciprocity-law failure probably due to limited rate of the ionic step. The emulsions containing the grains with high ionic conductivity showed no HIRF when they were chemically unsensitized, but they showed strong HIRF due to the dispersion of latent image when they were sulfursensitized. These facts were interpreted by the idea that small image centers were apt to be formed dispersedly when the time for the ionic step was comparable to the electron residence time in the temporary traps.