DIFFRACTIVE PRODUCTION OF VECTOR MESONS AND THE GLUON AT SMALL x

Perturbative QCD calculations of diffractive production of vector mesons are discussed. The MRT approach which uses k_T-factorization and Parton Hadron Duality is explained in some detail, and the uncertainties of the predictions are scrutinized. Skewing corrections turn out to be important. With recent data from HERA the strong sensitivity on the input gluon distribution at small x and small to intermediate scales will allow to better constrain the gluon.