CMS ECAL Laser Monitoring System

The CMS Electromagnetic CALorimeter (ECAL) is a precision detector made using scintillating PbWO$_4$ crystals, capable of 0.5\% energy resolution. However, the crystals will undergo significant changes in transparency during LHC running, and thus a monitoring system is needed to track crystal response to maintain the excellent design resolution. We will discuss the design and performance of the laser monitoring system, including new results test beams with the endcap ECAL detectors.