[en] In this paper, the Color Glass Condensate, an effective field theory of QCD at small-x and the physics of gluon saturation has been reviewed. This is essential to describe the gluon content of a saturated hadron or nucleus in high energy. Emphasis would be on its application to high energy heavy ion collisions. In particular the Glasma phase, that precedes the formation of an equilibrated quark-gluon plasma in the heavy-ion collisions has been discussed. The talk ends by citing some recent theoretical developments