  We present an overview of theoretical aspects of the phenomenon of gluon
saturation in high energy scattering in Quantum Chromo Dynamics. Then we review
the state-of-the-art of saturation-based phenomenological approaches to the
study and characterisation of the initial state of ultra-relativistic heavy ion
collisions performed at RHIC and the LHC. Our review focuses mostly in the
Color Glass Condensate effective theory, although we shall also discuss other
approaches in parallel.
