ATLAS results on inelastic pp collisions and the underlying event

The measurement of the properties of proton-proton interactions at center-of-mass energies ranging from 900 GeV to 7 TeV in the ATLAS detector are presented for a well defined kinematic region. A measurement of the inelastic cross-section at sqrt{s)=7 TeV has been made using a simple counting method with the MBTS trigger. Charged particles distributions(unfolded and corrected from observed tracks for detector effects)as a function of transverse momentum, pseudo-rapidity and particle density are studied in an inclusive manner. Activity in the Underlying Event was studies using independent measurements from the tracking detectors and calorimeters. Results are compared to Monte Carlo models and will provide input to derive new Monte Carlo tunes to constrain model parameters.