The emergence of optical metamaterials, and NIMs in particular, has given rise to numerous unusual linear and nonlinear optical phenomena that cannot be realized in conventional materials. The constituent components of these materials, meta-atoms, can be designed to empower optical magnetic activity, to enable fundamentally new regimes of linear and nonlinear light-matter interaction, and even to manipulate an object's degree of visibility at optical frequencies. The fabrication of optical NIMs is quite challenging and requires approaches going beyond a straightforward scaling of meta-atom sizes from microwave to near-infrared and visible frequencies. Several proof-of-principle experiments reported over the last two years form the basis for building functional NIMs and the realization of new photonic devices. Nevertheless, there is a lot of room for improvement and, therefore, for both experimental and theoretical fundamental and applied studies. The ultimate goal from the experimental viewpoint is the fabrication of three-dimensional, low-loss, tunable and broadband NIMs. Owing to the complexity of NIMs, theoretical predictions and numerical analysis are the essential components of NIM research. The demonstration of NIMs and magnetic activity at optical frequencies has motivated re-consideration of almost all well-established linear and nonlinear optical phenomena that reveal themselves in unusual and often counter-intuitive ways in NIMs. Optical metamaterials, without doubt, constitute one of the most exciting areas of research in modern optics, that is likely to result in discoveries of new phenomena and the development of novel device applications for nanophotonics.