Readiness of the CMS Detector for First Data

The Compact Muon Solenoid Detector (CMS) completed the first phase of commissioning in September 2008. The detector, data acquisition and distribution, reconstruction and analysis chains were successfully commissioned in a first phase with cosmic ray triggers. On September 10, 2008 CMS captured the first events from the LHC beam. In the following few days, the experiment accumulated many beam-splash and beam halo events from circulating beams. After the LHC setback on September 19th, CMS went back to cosmics operation. Continuous running with full magnetic field and the tracker detectors in full swing allowed the collection of large samples of muon tracks to be used for alignment and calibration, and improved the overall stability and efficiency of data taking. We present results of the analysis of data from the three phases, which have enabled establishing good starting points for time and space alignment, and accuracy of detector measurements. The status of the detector, and prospects for the collider run in 2009-2010 are subsequently discussed.