ATLAS results on inelastic pp collisions and the underlying event

Several results from ATLAS on the properties inelastic p-p collisions and the underlying event are presented. A new measurement of the inelastic cross-section at sqrt{s}=7 TeV has been made using events counted by the Minimum Bias trigger. Charged particle distributions were measured using tracks reconstructed in the ATLAS inner detector. Activity in the Underlying Event was studied using independent measurements from the tracking detectors and calorimeters. Results are compared to Monte Carlo models and provide input to derive new Monte Carlo tunes to constrain model parameters.