Searches for the Higgs boson at LHC with the ATLAS Detector

The Higgs boson is the only particle missing to complete the successful description of the elementary ingredients of our world. As of yet, it escapes experimental detection despite the enormous worldwide efforts. The new Large Hadron Collider (LHC) at CERN will provide enough collision energy for its formation, if it exists. The large experiments already installed, are equipped with excellent detector capabilities in order to confirm (or exclude) its existence. Discovery examples for each mass region of the Higgs boson are given using the complete - "as built" - detector simulation and the latest theoretical cross-section calculations, as well as, optimized selection criteria and statistical treatment. Furthermore, the sensitivity for discovery of the Higgs bosons in the framework of the extended SM model, the MSSM, is shortly reviewed. Emphasis is given to the expected discovery potentials from the first years of running.