An overview of selective leaching of clay minerals is provided on the preparation and properties of activated clays and porous materials. For the selective leaching, an acid or alkali solution is reacted with clay minerals under various conditions. Activated clays are prepared by reaction with an acid solution to increase the specific surface area (S_BET) with minimal change in chemical composition. Solid acidity is generated by activation of 2:1 type clay minerals with an appropriate degree of Al substitution in the tetrahedral SiO₄ layers. Among various clay minerals, activated saponite shows a greater degree of solid acidity than activated mont- morillonite and is a candidate catalyst. Activation of clay minerals can increase the adsorption of various adsorbates. By comparison with the mild leaching treatment used to prepare activated clays, porous materials are prepared under strong leaching conditions. Leaching of clay minerals depends critically on starting materials, being faster in clay minerals with Al substitution in the SiO₄ tetrahedral layers than in those without Al substitution. Porous silicas are obtained by selective acid leaching of various clay minerals. Higher S_BETs are obtained by using clay minerals with lower SiO₂ content, at lower leaching temperature and lower sample/solution ratio. The micropores initially formed in the products by selective leaching of the octahedral layers and interlayer cations can later change to mesopores by modifycation of the polymerized SiO₄ tetrahedra from a layered to a framework-like structure. The formation of Q³ structure units in the products has the important effect of enhancing the S_BET of the products.