The data acquisition (DAQ) system of the CMS experiment at LHC performs the read-out and assembly of events accepted by the first level hardware trigger. Assembled events are made available to the high-level trigger which selects interesting events for offline storage and analysis. The system is designed to handle a maximum input rate of 100 kHz and an aggregated throughput of 100 GBytes/s originating from approximately 500 sources. An overview of the architecture and design of the hardware and software of the DAQ system is given. We discuss the performance and operational experience from the first months of LHC physics data taking. Presented at CHEP2010: International Conference on Computing in High Energy and Nuclear Physics 2010 The data-acquisition system of the CMS experiment