Mess is a substrate for building simulation environments suitable for testing plans and on-line or realtime planners. The article describes the design of Mess, how simulations are built and how on-line planners integrate with the substrate. Mess supports activities , defined as processes over some time interval, and interactions between activities and other simulation events. Mess interfaces with TCL , which is a portable, extensible definition of computation time, enabling Mess to be used for platform-independent simulations of real-time planning. Mess has been used to re-implement the Phoenix testbed, which simulates forest fires and planning for firefighting agents.