DISCOVER: An environment for Web-based interaction and steering of high-performance scientific applications. This paper presents the design, implementation, and deployment of the DISCOVER web-based computational collaboratory. Its primary goal is to bring large distributed simulations to the scientists’/engineers ’ desktop by providing collaborative web-based portals for monitoring, interaction and control. DISCOVER supports a 3-tier architecture composed of detachable thin-clients at the frontend, a network of interactions servers in the middle, and a control network of sensors, actuators, and interaction agents at the back-end. The interaction servers enable clients to connect and collaboratively interact with registered applications using a browser. The application control network enables sensors and actuators to be encapsulated within, and directly deployed with the computational objects. The application interaction gateway manages overall interaction. It uses Java Native Inte! rface to create Java proxies that mirror computational objects and allow them to be directly accessed at the interaction server. Security and authentication are provided using customizable access control lists and SSL-based secure servers.