  I review the physical and mathematical foundations for the theoretical
description of the hadron wavefunction at small $x$ as a Colour Glass
Condensate. In this context, I discuss the phenomenon of gluon saturation and
some of its remarkable consequences: a new ``geometric scaling'' for $F_2$,
which has been recently identified at HERA, and the unitarization of the
hadronic cross-sections at high energy. I show that by combining saturation and
confinement one obtains cross-sections which saturate the Froissart bound.
