Funded by the National Science Foundation (NSF), TeraGrid is one of the world's largest distributed cyberinfrastructures for open scientific research. The project began in 2001 as the Distributed Tera-scale Facility, which linked computers, visualization systems, and data at four sites through a dedicated 40-gigabit optical network. Today TeraGrid includes 25 platforms at 11 sites and provides access to more than a petaflop of computing power and petabytes of storage. TeraGrid has three primary focus areas. Its deep goal is to support the most challenging computational science activities those that cannot be achieved without TeraGrid facilities. TeraGrid's wide mission is to broaden its user base. The project's open goal is to achieve compatibility with peer grids and information services that allow development of programmatic interfaces to TeraGrid. The Science Gateways program seeks to provide researchers with easy access to TeraGrid's high-performance computing resources. A look at four successful gateways illustrates the program's goals, challenges, and opportunities.