Jet Photoproduction and QCD at Hera.

Dijet production by almost real photons has been studied at HERA with the ZEUS detector. Jets have been identified using the cone algorithm. A cut on x _sp{gamma}{OBS}, the fraction of the photon energy participating in the production of the two jets of highest transverse energy, is used to define cross sections that not only able to test perturbative QCD in a new kinematic regime, but are also sensitive to the parton distributions in the proton and in the photon. The dependence of the dijet cross sections on the average pseudorapidity of the two jets has been measured for x_sp{gamma} {OBS}>=q0.75 and x_sp {gamma}{OBS}<0.75. The former is constructed to be sensitive to the gluon momentum density in the proton; the latter, sensitive to the gluon in the photon. The cross sections are corrected for detector acceptance and compared to leading order QCD calculations.