In the case of metal NP synthesis is carried out with a sol method by using capping agents to prevent aggregation and control the size. Quantum confinement occurs when the electrons motion is limited by the size of the NP. Here the electronic energy levels of the particle around the Fermi level are affected only for very small sizes. The UV-visible absorption is determined by the surface plasmon resonance, which is size and shape dependent. These exhibit a very weak luminescence and the I-V behaviour is characterized by the Coulomb blockade. These show valuable catalytic properties owing to the large surface to mass ratio. Semiconductor NP synthesis is carried out with a variety of chemical methods to produce the desired size, shape and structure. Here quantum confinement occurs when the radius of the NP is comparable to the exciton’s Bohr radius. The electronic energy levels of the particle around the Fermi level are strongly affected for sizes in the 10-100 nm range. The UV-visible absorption is determined by the band edge transition, which is strongly size and shape dependent (bandgap energy increases as the size decreases) and hence highly tunable. These can exhibit band edge or defect (redshifted) luminescence, in both cases luminescence can be very intense and can be tuned. The I-V behaviour of these material is characterized by the Coulomb blockade. They show valuable catalytic properties, owing to the large surface to mass ratio.