  Quantum systems driven by a time-periodic field are a platform of condensed
matter physics where effective (quasi)stationary states, termed "Floquet
states", can emerge with external-field-dressed quasiparticles during driving.
They appear, for example, as a prethermal intermediate state in isolated driven
quantum systems or as a nonequilibrium steady state in driven open quantum
systems coupled to environment. Floquet states may have various intriguing
physical properties, some of which can be drastically different from those of
the original undriven systems in equilibrium. In this article, we review
fundamental aspects of Floquet states, and discuss recent topics and
applications of Floquet states in condensed matter physics.
