Self-assembled clusters of nanoparticles constitute a class of hybrid nanostructures that exhibit novel functionalities. The clustering often results in an enhancement of the functionalities or combination of different functionalities. For instance, clustering of plasmonic nanoparticles facilitate coupling of the individual plasmon modes of the nanoparticles to result in an enhancement in the light-nanoparticle interactions. Similarly, the combination of a plasmonic nanoparticle with a catalytic nanoparticle results in clusters with photocatalytic functionality which is enhanced by the plasmonic component. However, uncontrolled aggregation of nanoparticles may lead to precipitation or polydisperse clusters, which is not desirable in most cases. In a colloid, as nanoparticles have high surface energy due to the small size, they tend to aggregate by Van der Waals attractive interactions. On the other hand, the surface charge repulsion or steric hindrance due to stabilizing surface molecules prevent the aggregation of the nanoparticles . The DLVO (Derjaguin-Landau-Verwey-Overbeak) theory combines Van der Waals attractive force and electrostatic repulsive force due to the electric double layer (EDL) to predict the overall force between particles. The repulsive forces pose a barrier as electrical repulsive forces drop exponentially, while the Van der Waals forces decrease are inversely related to the square of the interparticle distance. It is the height of the energy barrier that determines if the collision energy can exceed it to promote aggregation. The variation of pH and dissolved ionic solutes can thus easily lead to the point of zero charge (PZC) or zero zeta potential, which leads to fast aggregation of nanoparticles without requiring large supply of external energy. This kind of clustering is perhaps the simplest assembly process as citrate (or even ligand stabilized) stabilized nanoparticles can be assembled by the addition of normal salt that neutralizes the zeta potential. [13] The field of self-assembly has evolved with different strategies to obtain more complex nanostructures with greater control. In what follows, different self-assembly methods to synthesize finite clusters of nanoparticles and cluster arrays are discussed.