  The Vertex Locator (VELO) is a silicon microstrip detector that surrounds the
proton-proton interaction region in the LHCb experiment. The performance of the
detector during the first years of its physics operation is reviewed. The
system is operated in vacuum, uses a bi-phase CO2 cooling system, and the
sensors are moved to 7 mm from the LHC beam for physics data taking. The
performance and stability of these characteristic features of the detector are
described, and details of the material budget are given. The calibration of the
timing and the data processing algorithms that are implemented in FPGAs are
described. The system performance is fully characterised. The sensors have a
signal to noise ratio of approximately 20 and a best hit resolution of 4
microns is achieved at the optimal track angle. The typical detector occupancy
for minimum bias events in standard operating conditions in 2011 is around
0.5%, and the detector has less than 1% of faulty strips. The proximity of the
detector to the beam means that the inner regions of the n+-on-n sensors have
undergone space-charge sign inversion due to radiation damage. The VELO
performance parameters that drive the experiment's physics sensitivity are also
given. The track finding efficiency of the VELO is typically above 98% and the
modules have been aligned to a precision of 1 micron for translations in the
plane transverse to the beam. A primary vertex resolution of 13 microns in the
transverse plane and 71 microns along the beam axis is achieved for vertices
with 25 tracks. An impact parameter resolution of less than 35 microns is
achieved for particles with transverse momentum greater than 1 GeV/c.
