The Energy Sciences Network is the Department of Energy's high-speed network that provides the high-bandwidth, reliable connections that link scientists at national laboratories, universities and other research institutions, enabling them to collaborate on some of the world's most important scientific challenges. Managed and operated by the ESnet staff at Lawrence Berkeley National Laboratory, ESnet provides direct connections to more than 40 DOE sites at speeds up to 100 gigabits per second, allowing scientists to manage, share and analyze massive datasets that are the hallmark of 21st century science. Connectivity to the global Internet is maintained through "peering" arrangements with more than 100 other research and education networks. Funded principally by DOE's Office of Science, ESnet allows scientists to use unique DOE research facilities and computing resources independent of time and location with state-of-the-art performance levels. ESnet derives its effectiveness from the extensive cooperation it enjoys with its user community. It is one of the most widely based and successful cooperative efforts within the Department of Energy. ESnet also operates three national-scale network research testbeds, available to researchers in the commercial, non-profit, university, or federal sectors.