Publisher Summary This chapter explains the role of the Online Book-Keeper (OBK) as one of the main collectors and managers of metadata produced online, how that data is stored, and the interfaces that are provided to access it—merging the physics data with the collected metadata will play an essential role for future analysis and interpretion of the physics events observed at ATLAS. The OBK is a component of ATLAS' Online Software—the system that provides configuration, control and monitoring services to the DAQ (Data AQquisition system). ATLAS is one of the four detectors being built for the Large Hadron Collider (LHC) particle accelerator at CERN to be completed in 2006. After being accelerated, particles will collide inside the detector. From observing, the results of those collision physicists expect to be able to expand the knowledge on the basic contituents of matter.