  Equilibrium statistical mechanics rests on the assumption of ergodic dynamics
of a system modulo the conservation laws of local observables: extremization of
entropy immediately gives Gibbs' ensemble (GE) for energy conserving systems
and a generalized version of it (GGE) when the number of local conserved
quantities (LCQ) is more than one. Through the last decade, statistical
mechanics has been extended to describe the late-time behaviour of periodically
driven (Floquet) quantum matter starting from a generic state. The structure
built on the fundamental assumptions of ergodicity and identification of the
relevant "conservation laws" in this inherently non-equilibrium setting. More
recently, it has been shown that the statistical mechanics has a much richer
structure due to the existence of {\it emergent} conservation laws: these are
approximate but stable conservation laws arising {\it due to the drive}, and
are not present in the undriven system. Extensive numerical and analytical
results support perpetual stability of these emergent (though approximate)
conservation laws, probably even in the thermodynamic limit. This banks on the
recent finding of a sharp ergodicity threshold for Floquet thermalization in
clean, interacting non-integrable Floquet systems. This opens up a new
possibility of stable Floquet engineering in such systems. This review intends
to give a theoretical overview of these developments. We conclude by briefly
surveying the experimental scenario.
