Tetrahedral clusters possessing two or three organometallic vertices, such as Co(CO) 3 or CpMo(CO) 2 , provide enormously enhanced stability to neighboring cationic centers. The factors controlling the interactions between the electron-deficient centers and the metals are discussed in terms of their frontier molecular orbitals. Rational syntheses of these systems are now available, and many such metal-stabilized cations have been isolated and characterized not only spectroscopically but also by X-ray crystallography. These species exhibit a variety of fluxional processes which have been elucidated by variable-temperature nuclear magnetic resonance (NMR) measurements and also by a Burgi-Dunitz analysis of a series of closely related X-ray crystal structures. By preparing clusters containing different organometallic fragments, it is possible to establish a hierarchy of moieties best able to alleviate the electron deficiency at the carbenium ion center. Finally, the growing usage of these metal-stabilized carbocations in organic synthesis, in the elucidation of reaction mechanisms and even in biological chemistry is described.