Temporal modulation of material parameters provides a new degree of freedom for metamaterials, metasurfaces and wave-matter interactions as a whole. In time-varying media the electromagnetic energy may not be conserved, and the time reversal symmetry may be broken, which may lead to novel physical effects with potential applications. Currently, theoretical and experimental aspects of this field are rapidly advancing, expanding our understanding of wave propagation in such complex spatiotemporal platforms. This field promises novel possibilities and directions in research, innovation and exploration.