1. Definition of nanocomposites Nanocomposites are a novel class of composite materials whose reinforcements have dimensions in the range of 1-100 nm. Although nanoscale reinforcements (or nanofillers) of nanocomposites have different kinds of fillers such as nanofibers, nanowires, nanotubes and nanoparticles etc, their mechanical behaviors have some common features. Figure 1 shows a potential use of nanocomposites as multifunctional materials. Since many important chemical and physical interactions are governed by surfaces and surface properties, and nanoscale reinforcements have a large surface area for a given volume, nanocomposites are ideal multifunctional materials. For nanotube-reinforced materials, coupled mechanical and electric properties of nanotubes can be used for very small-scale health monitoring.