  We overview the current status and recent developments on initial conditions
in ultra-relativistic nucleus-nucleus collisions. Specifically, we look at the
progress in understanding the role of sub-nucleonic fluctuations in large and
small collision systems. Next, we review the current ideas of going beyond
boost invariant initial conditions and introducing physically motivated
rapidity fluctuations at RHIC and LHC energies. Finally, we discuss the time
evolution of initial stages and the matching between different descriptions of
the QGP.
