This chapter discusses mixed-metal clusters, methods of characterization, reactivity, and dynamic nuclear magnetic resonance (NMR) studies. Transition-metal cluster compounds are currently under intensive scrutiny because of their potential catalytic applications, both as models for understanding catalytic metal surfaces and as catalysts in their own right. Noticeably, few metal clusters have been prepared by designed or rational synthetic procedures. Pyrolysis reactions generally involve heating together two or more stable compounds of different metals, presumably to give fragments that then combine to yield the mixed-metal clusters. Relatively few clusters have been prepared by the pyrolysis of two monomeric compounds, and only two examples are given. Metal–carbonyl dimers have proved to be useful reagents for the synthesis of mixed-metal clusters. The pyrolysis of clusters in the presence of monomers, dimers, or other clusters usually requires much more severe reaction conditions. These coordinatively unsaturated species are apparently the key intermediates that condense to give the cluster products. The reaction of carbonylmetalates with monomeric and dimeric carbonyls has yielded many mixed-metal clusters. The reaction of carbonylmetalates with trinuclear clusters provides, in many cases, a convenient synthesis of tetranuclear clusters. Carbonylmetalates displace a halide from a metal–halide complex to yield a metal–metal bonded species.