Charm and beauty structure of the proton

This paper will review the data collected by the H1 and ZEUS Collaborations concerning the production of heavy quarks (charm and beauty) at the electron-proton collider HERA. Heavy quark production is an important testing ground for quantum chromodynamics (QCD), because QCD calculations are expected to be reliable if a hard scale is present in the process. In heavy quarks production a hard scale is provided by the quark mass. Moreover heavy quarks production can give direct access to the gluon density in the proton due to the fact that it proceeds, in QCD, almost exclusively via photon-gluon fusion, where a photon from the incoming electron interacts with a gluon in the proton giving an heavy quark-anti-quark pair. Results will be shown both for deep-inelastic scattering (DIS), where the virtuality of the exchanged boson Q**2 is large, and photo-production, where the Q**2 is equal to zero. Various experimental techniques are used in order to select charm and beauty events, ranging from the measurement of D* cross section to impact parameter analyses. The results are found to be compatible with the predictions of perturbative QCD. A brief summary of the open charm analyses done by the COMPASS Collaboration will be also given.