As Grids grow in size, more emphasis is being placed on the discovery of available resources and the monitoring of their status. The Globus Toolkit Monitoring and Discovery System (MDS4) provides resource selection and error warning support to large scale deployments. It offers a wide variety of information sources, from any WSRF service to basic executables to the ability to scrape files for data, higher-level services, including both an Index service and a Trigger warning service, and several interfaces, not just Java and command line but flexible GUIs based on XSLT transforms. This poster details the basic architecture and several current deployments, including TeraGrids' use of MDS4 for resource discovery and the Earth System Grid's use of MDS4 to do error reporting and trouble shooting.