Gluon Saturation

This is a review on gluon saturation. Sec. 1 gives a general introduction to the subject while Sec. 2 has a brief discussion of quark saturation. Sec. 3 covers gluon saturation in a semiclassical limit and the much more general context of QCD evolution. Surprisingly gluon occupancies in a high energy hadron can become arbitrarily large, much larger than $1/\alpha$. Sudakov effects are introduced in Sec. 4 and reduce the maximum gluon occupancies to less than or equal to $(1/\alpha)^{3/2}$ in both fixed coupling and running coupling QCD. In Sec. 5 the role of saturation in coherent states in QCD is reviewed as well as brief discussions on nuclear shadowing, heavy ion collisions and Drell-Yan like processes are given.