Nanosciences and nanotechnology (NST) constitute currently a major research field all over the world. NST deal with the study of phenomena and manipulation of materials at atomic, molecular and macromolecular scales, where properties differ significantly from those at the larger scale. The properties of materials can be different at the nanoscale for two main reasons : size and quantum effects. Effects negligible at the macroscopic level become important at the nanometer scale, and vice versa. Scaling laws are described in order to understand some differences. Moreover, geometric arguments are necessary to understand the origin of some physical and chemical properties of nanosystems. On the other hand, quantum effects can begin to dominate the behaviour of matter at the nanoscale – particularly at the lower end – affecting the optical, electrical and magnetic behaviour of materials. The characteristic dimensions for which the properties change from the “macro-“ to the “nano-“ regimes are discussed.