The Selective Service is a system in the United States that registers young men for potential military conscription, or the draft, in case of a national emergency. All male citizens and immigrants residing in the U.S. must register when they turn 18, ensuring that the government can quickly mobilize forces if needed. Registration with the Selective Service System is mandatory, and failure to register can lead to penalties, including fines and ineligibility for federal jobs or student financial aid. The system has been in place since 1917 and plays a crucial role in maintaining national security readiness.