The authors outline the basic principles first, illustrating these by a variety of experimental and theoretical findings. Several topics related to the zeolite framework and its structure and reactivity are compiled. The different structure types, symmetries, synthesis mechanisms and framework modifications are discussed from a structural-chemical viewpoint. The properties of the Si-O and Al-O bonds, and how these are modified by the structure types and by interactions with exchangeable cations, are stressed. Two concepts facilitating the general understanding are introduced: the principle of electronegativity equalization and a structural-chemical approach to molecular interactions. At this point also, the different factors influencing the 'equilibrium' geometry of the zeolite framework are illustrated. Physical measurements are discussed which give direct insight into the cation-framework interactions. Two extremely important cations, Al**3** plus in cation-exchange sites and protons, are treated. The chemistry of transition metal ion complexes in zeolites is discussed. The following aspects are emphasized: introduction of complexes in the cavities or channels; their physical characterization; the decomposition in inert, oxidizing or reducing atmospheres; the activation of small molecules and the catalysis.
