Valuable information on the structure and formation of metal-polymer interfaces originates from radiotracer measurements of metal diffusion at the interface, structural investigations by means of transmission electron microscopy, and computer simulations on the interplay of atomic metal diffusion and aggregation. Moreover, X-ray photoemission spectroscopy has largely contributed to our present understanding of the interfacial chemistry and the early stages of interface formation. While reactive metals always form relatively sharp interfaces with polymers, metals of lower reactivity diffuse into polymers at elevated temperatures and have a very strong tendency to agglomerate. The extent of diffusion appears to be determined by the initial stage of the deposition process. Here sticking coefficients recently measured for metals on virgin polymer surfaces deviate markedly from unity. Diffusion into the polymer increases strongly at low deposition rates. No significant diffusion is expected from a continuous metal film unless metal ions are formed at the interface. Metal ions are highly mobile and do not aggregate due to electrostatic repulsion. The model emerging from these observations allows us to predict the salient features of interface formation between metals and polymers in general and particularly with respect to the new low-k polymers.