High-$p_T$ results and parton density functions from ATLAS.

The ATLAS detector at the LHC is an ideal experiment to rigorously test QCD in a large variety of final states. This talk will focus on recent results over a wide range of energies, which are particularly sensitive to parton density functions. We will review results with high-$p_T$ jets, photons and vector bosons. We will then discuss fits to determine parton distribution functions (PDFs) using these diverse sets of measurements from the ATLAS experiment. These ATLAS measurements are used in combination with deep-inelastic scattering data from HERA. Particular attention is paid to the correlation of systematic uncertainties within and between the various ATLAS data sets and to the impact of model, theoretical and parameterisation uncertainties. Finally, we will also present results on the determination of $\alpha_s$