Conventional inorganic polymers, otherwise known as geopolymers, are aluminosilicates with the desirable properties of ceramics (durability, strength) but which harden at ambient temperatures without the need for processing at high temperatures. They are commonly formed by the action of alkalis or phosphoric acid on aluminosilicate minerals such as dehydroxylated clays and consist of randomly arranged three-dimensional assemblages of silicate and aluminate units into an X-ray amorphous structure. The presence of charge-balancing ions such as Na⁺ or K⁺ within this structure gives them useful ion-exchange properties similar to zeolites. Analogous inorganic polymers are known in which the silicate is partially replaced by germinate units, and the aluminate units are replaced by gallate, phosphate, or borate units, giving a range of materials with a variety of properties and potential applications. The physical properties of the inorganic polymers can be manipulated by the introduction of fibers, whereas their chemistry allows the introduction of bone-forming elements, drugs and carbon nanotubes, providing them with porosity, bioactivity, or electronic functionality. Inorganic polymers with catalytic, photocatalytic, and fluorescent functionality can also be produced. This article covers the synthesis and structure of clay-based inorganic polymers and discusses a number of their more specialized applications. Although inorganic polymers are a subclass of alkali-activated materials that can also be formed by a range of materials other than dehydroxylated clays (fly ash, volcanic ash, blast furnace slag), geopolymers formed from these materials are outside the scope of this article, and are not discussed here.