Over the last decade, much attention has been paid to the field of “nanocatalysis”. Within this new field the unique characteristics of metal clusters (Au, Ag, Cu, Pd, etc.) make them strong candidates to replace the old catalysts. Metal clusters (CLs) can be defined as aggregates of metal atoms with sizes below approximately 1 to 2 nm.¹  They represent a novel state of matter, located between the classical bulk (or nanoparticle) behaviour and the different behaviour of the corresponding atoms. Different studies show that the new electronic/geometrical properties displayed by these atom clusters depend on their size due to quantum confinement. The quantized electronic structures of clusters and the extraordinary high stability of magic-number clusters, M_N (N=8, 18, 20, 34, 58…) due to electronic shell closure,²  could explain the chemical reactivity and novel properties of these clusters. One of the most important consequences of this regime is the presence of discrete energy levels and the appearance of a HOMO–LUMO gap (highly dependent on the cluster size, increasing with decreasing size) similar to that found in semiconductors (Figure 8.1). It has to be pointed out that this size-dependence gives the chance of obtaining different band gaps with only one metal element, and therefore, different specific clusters of the same metal could be used for different catalytic applications.³  The discrete electronic states in clusters provide very different properties than those of their counterparts (atoms and bulk/nanoparticles), such as well-defined luminescence, magnetism and catalysis,⁴  among others, providing a wide range of attractive applications,⁵  like in biological labelling, optical sensing, etc. The correlation of the catalytic properties of metal clusters with their atomic structure is still under investigation and will provide important guidelines for the future design of new catalysts for specific chemical processes.