Performance of the LHCb detector during the LHC proton runs 2010 – 2012

The LHCb experiment at CERN's Large Hadron Collider (LHC) searches for New Physics through precision measurements in the domain of heavy flavour physics, exploiting in particular the large B hadron production. After a short introduction to the requirements of the LHCb detector, this paper reviews the operational strategy during the first three years of data taking. The focus is on the detector performance together with a description of several fundamental system developments which emerged in this period and which allowed LHCb to venture well beyond its design parameters and to extend the physics program. With the inclusion of a solid charm physics program and electroweak and soft QCD measurements in the forward direction, LHCb has established itself as an excellent forward general purpose detector at the LHC. Running at twice the design luminosity, LHCb has been able to collect an integrated luminosity of more than 3 fb-! at an operational inefficiency of less than 4% in the first three years.