This paper describes the nature of nanometer particles involving considerations of the kinetic theory of gases, quantum mechanics, and aerosol dynamics. It is shown that particles in the size range below about 5 nm contain only few molecules with a high proportion at the surface. Hence, the structure of such particles cannot be regarded as a continuum and the traditional concepts of particle volume and surface area are no longer prime parameters. The electronic state of the particle is determined by the electronic states of the individual constituent molecules. Their configuration is extremely important in the particle interactions with other entities, including molecules, other particles, and biological cells. It is acknowledged that the physics of very, very small particles and their interactions are currently understood only for very simple systems. But, in view of the profound implications for many areas of practical interest, it is expected that during the years ahead multidisciplinary research will lead to rapid advances in our understanding.