ATLAS detector commissioning results with cosmics and first beam

An intense finalization and commissioning activity has preceded the delivery of the first LHC events to its experiments. Both cosmic rays and LHC beams have demonstrated to be important tools for the understanding and tuning of our detectors performance in terms of calibration but also as a training ground for the operation of such a complex machinery. After a brief introduction, this paper will discuss the ATLAS detector readiness status and some aspects of its recent commissioning results after the commissioning campaigns of 2008 and 2009. Emphasis will be put on topics relevant for B physics performance.