"Clusters behave as a link between the atom and the bulk material. They exhibit characteristics of both forms of matter, depending on their size and their molecular and electronic structures. Such molecules can be potentially useful for technical applications as specific and very efficient catalysts, drugs, and other novel materials with as yet unimagined properties. As an example, the continued miniaturization of electronic devices is pushing towards the nano- and molecular scales. In this sense, silicon clusters can be very important in the future development of the electronic industry. Other clusters are expected to have relevant catalytic or magnetic properties. All-metal and semi-metal clusters have been recently discovered. The unusual stability of all these clusters comes from their aromatic character. Indeed, the aromaticity is a key property of these compounds since it explains their stability and reactivity. As compared to the classical aromatic organic molecules, that possess only π-electron delocalization, the aromaticity in inorganic clusters is more complex. These compounds have σ- and π- (or even δ-) electron delocalization, thus giving rise to the so-called multifold aromaticity. All-metal and semimetal aromatic clusters represent one of the “new frontiers” of chemistry (the first of such clusters was discovered in 2001). It is expected that understanding the origin of aromaticity in these systems will help us to comprehend the factors that govern structural patterns and stability in solids. In addition, these novel materials are expected to have as-yet unimagined properties such as, for instance, nanodevices, intense non linear optical properties, high catalytic power and efficiency, and utility as possible drugs."