The most common and prototypical nanoporous materials are represented by the zeolites. Zeolites are crystalline solids with well-defined structures that generally contain silicon, aluminium and oxygen in their framework and cations, water and/or other molecules wthin their pores A defining feature of zeolites is that their frameworks are made up of 4-connected networks of atoms. It has a tetrahedra, with a silicon atom in the middle and oxygen atoms at the corners. These tetrahedra can then link together by their corners to from a rich variety of beautiful structures. The framework structure may contain linked cages, cavities or channels, which are of the right size to allow small molecules to enter - i.e. the limiting pore sizes are roughly between 3 and 10 Å in diameter. In all, over 130 different framework structures are now known. In addition to having silicon or aluminium as the tetrahedral atom, other compositions have also been synthesised, including the growing category of microporous aluminophosphates.