Understanding the nanoscopic properties of solids at the fundamental nanoscale level is the main objective of this chapter. Considering the multiple particles present in solids, developing a nanoscopic explanation is a daunting endeavor. The classical or quantum–mechanical equations of motion of the particles are obviously unsolvable. Despite the enormous number of particles involved, it turns out that materials are frequently crystalline, with the atoms arranged on a regular lattice. This symmetry enables us to identify nanoscopic models. Despite the impression that these flawless crystals have no bearing on actual materials, this is untrue. Small crystalline grains make up the composition of many substances. Molecular organic crystals can be found in a wide range of material science fields. Most small-molecule medications are taken orally in the form of crystals. Molecular crystals are essential parts of pigments, insecticides, and fertilizers. They can be used as sensors, photovoltaic cells, field effect transistors, and light-emitting diodes.