Charmonium production in ep collisions at HERA

New results are presented on the production of J/ψ mesons in electron-proton collisions, measured with the ZEUS and H1 detectors at HERA. The J/ψ mesons are detected by their leptonic decay modes into electrons and muons. The results on inelastic J/ψ production are interpreted in the framework of the colour singlet model while elastic J/ψ production is compared with diffractive models based on pomeron exchange. The photoproduction cross-section for elastic J/ψ meson production is observed to rise with the centre-of-mass energy, faster than expected from the exchange of a soft pomeron. The energy dependence of the cross-section for inelastic production agrees in shape with the NLO QCD calculations based on the colour singlet model, if the gluon distribution in the proton is assumed to rise strongly towards low x