Natural amorphic silicas were charactrized, present in the form of minerals. Subsequently, synthetic products based on the amorphic silica were described, including, so called, colloidal silica (silica gels, xerogels, aerogels, pyrogenic silicas, precipitated silicas and coacervate silicas). Particle structure of the amorphic colloidal silica was presented and compared to that of silica crystals (e.g. of quarz crystals). Particular attention was given to the outer structure of silica colloidal particles and two types of silanol groups, present on the surface of the particles. Methods of the surface stabilization in the alcaline medium were indicated. Subsequently, effects of temperature on the amounts of individual types of silanol and siloxane groups on the surface of colloidal silicas were discussed. In the next chapter of the presentation physical and selected chemical properties of the silicas were presented. Attention was payed also to the most significant issue of stabilization of silica colloidal systems.