One of the most intriguing properties of optical metamaterials formed from conducting or semiconducting materials is their ability to localize and enhance optical fields at selected “hotspots” within the metamaterial structure. Such optical hotspots can be used to enhance the nonlinearities associated with materials embedded within the hotspots—or even enhance the intrinsic nonlinearity of the metals that form the metamaterial or plasmonic structure. Understanding the underlying mechanisms of optical nonlinearity in metals as well as nonlinear enhancement is one of the major thrust areas within CMIP, requiring a combination of theory, modeling and experimental investigation. The exploitation of nonlinear enhancement and wave propagation will lead to new photonic devices, such as all-optical switching and computing, as well as many other applications relevant to imaging, communications and networking.