Publisher Summary This chapter deals with the catalytic properties of isolated transition metal ions connected to the framework of a molecular sieve. It focuses on the materials that are active catalysts in oxidation reactions, which include titanium, vanadium and chromium silicalites, as well as cobalt-, vanadium-, and chromium-substituted AlPOs. The metals in this list are known to catalyze a host of different oxidation reactions in which different mechanisms are operating, some with electron transfer steps and autoxidation chains, others with heterolytic or homolytic oxo or peroxo oxygen transfer steps. Although the mechanisms have little in common, they are all likely to involve ligand exchange and non-tetrahedral coordination states, perhaps accompanied by reversible framework bond lysis. The chapter discusses that the resistance of the titanium site to extensive hydrolysis imposes restraints on the accessibility of the site; as a consequence, the site is apparently sufficiently electron-deficient to be a very active catalyst for oxygen transfer from hydrogen peroxide. Together with those properties, site separation and selective sorption by the molecular sieve matrix explains several features of the catalytic chemistry of titanium silicalites. A better insight allows predicting to some extent whether it is possible to synthesize catalysts based on other molecular sieves or containing other transition metals.