Upon reaching the age of eighteen, young men are required by federal law to register with Selective Service. The Selective Service System is not part of the Department of Defense. It is a separate, independent government agency whose purpose is to keep track of men between the ages of eighteen and twenty-five in the event they are needed to serve the country in an armed conflict or other crisis. All male residents of the United States, including citizens, immigrants with work visas or green cards, and undocumented aliens are required to register with Selective Service. Forms are available at any post office, and it is also possible to register online by visiting the Selective Service Web site, at www.sss.gov.