Metamaterials are artificial electromagnetic media that are structured on the subwavelength scale. Such structures were initially suggested for achieving the negative index of refraction, but later they became a paradigm for engineering electromagnetic space and controlling propagation of waves through the transformation optics and optically-induced magnetic response. The research agenda is now shifting towards tunable, switchable, nonlinear and sensing functionalities of metamaterials and their applications, and it involves the fields of metasurfaces and metadevices, with the recent demonstrations of breakthrough results with light-controlled metamaterials, nonlinear metamaterials, and tunable metasurfaces for MRI applications. Here, we briefly discuss the basic concepts of this rapidly growing research field, highlighting the recent developments in the physics of metamaterials, subwavelength nanophotonics, and graphene-based photonics.