The CMS Fast Beam Condition Monitor (BCM1F) is a subdetector dedicated to the online measurement of the luminosity and to the monitoring of beam-related background rates. BCM1F consists of 24 sensors (diamond and silicon) of 5×5 mm2 area each, mounted on two rings close to the beam pipe at a distance along the beam pipe of 1.8 m on each side of the interaction point. Due to the excellent time resolution of the device, signals from proton-proton collisions can be distinguished from background signals on a bunch-by-bunch basis using their arrival time. By use of a fast real-time histogramming unit, BCM1F delivers a measurement of the instantaneous luminosity and the background rate to the CMS and LHC control rooms, independently of the operational state of the CMS experiment. It also provides a fast beam-abort system for protection of the CMS track detectors. The DESY group plays a leading role in the operation of the device, including commissioning, calibration and day-to-day operation.