The chemistry of transition metal complexes in zeolites has received considerable attention in our laboratory because we believe that such complexes have potential both in fundamental and applied catalysis. To scientists outside of the field of heterogeneous catalysis it often appears virtually hopeless to obtain meaningful chemistry from surface reactions because the active centers are so poorly characterized. Although most catalytic chemists would not totally agree with this viewpoint, one would have to admit that it contains an element of truth. The traditional approach to solving this problem has been to attempt better characterization of active sites on known catalysts, and while some significant advances have been made in this direction, the progress in general has been painfully slow. Another approach, and the one which will be described here, has been to synthesize surface complexes which are both well defined and active as catalysts. These then may serve as model systems for conventional catalysts, but more than this, they may be unique catalysts which are important in their own right.