11.1 intrOductiOn Geographic information systems (GISs) are used to support water resources hazards monitoring, forecasting, and warning in a variety of ways. Central to monitoring and warning is the need for data collection in real time when the event is happening and to anticipate future conditions through fore- casting. These GISs are driven by advanced data-collection systems that measure the full range of hydrological and related system parameters, including water flow and weather. Telecommunications of data by various modes, in real time, provide up-to-date databases required for timely and accu- rate control and analyses. GIS-based interactive displays allow water resource systems managers to quickly examine the state of the system, invoke models to forecast possible future conditions, and disseminate warnings. The intent of this chapter is to summarize technologies for real-time data collection and transmis- sion as well as the monitoring and forecasting systems that utilize these data. Emphasis is on flood warning systems and general concepts of flood warning/preparedness programs, flood-threat recog- nition, warning dissemination, emergency response, and plan management. The concepts of moni- toring and forecasting apply to other resource sectors having less time-critical response factors, such as droughts. GIS concepts and tools play a central role in all aspects of data collection and archiving, support for condition assessment through display and modeling, and message dissemination.