The Selective Service System (SSS) is an independent American agency which registers all men between the ages of 18 and 25, in the event that the United States Congress authorizes a draft for military service. The SSS also maintains a database of healthcare workers, in the event that people in this vital field are needed. The Selective Service System claims to be able to have inductees ready within 193 days of a draft authorization.

This agency was founded in 1917 by an act of Congress which was passed in response to concerns about manpower for the First World War. It is responsible for registering all eligible American men, establishing a lottery system if a draft is authorized, calling men by lottery number for physical examinations to determine whether or not they are fit to serve, and processing deferment requests and conscientious objector applications. The system ensures that the United States will have manpower available in the event of a major military conflict.

By law, every American male must register within 30 days of his 18th birthday, regardless as to intent to apply for conscientious objector status or physical condition. Women are not allowed to register, a policy which has been criticized by some people. The practice of conscription is somewhat controversial in general in the United States, causing some people to resent the Selective Service System. Some people choose not to register, potentially facing prison time and fines. The Selective Service System also penalizes people who do not register by restricting their access to federal jobs and financial aid for education.

In the event that a draft is authorized, the Selective Service System is able to mobilize a large number of regional draft boards which remain dormant until they are needed. These boards oversee the administration of the draft in their regions, and they process deferments and other issues which arise in the course of mobilization. When a draft is authorized, the Selective Service System sets up a lottery based on birthday, calling 20 year olds first and then 21, 22, 23, 24, 25, 19, and 18 year olds, respectively.

When a man's lottery number comes up, he must report to a regional draft board. The draft board administers a physical examination and also evaluates potential inductees for mental and moral fitness to determine whether or not they would be a good fit with the military. After this evaluation, the man is given a classification such as 4F for unfit, or 1A, meaning that he is available for service. The potential inductee has 10 days to appeal the classification or he will be required to report for service.

Appeals can take a number of forms. Some people are automatically exempt from service, such as ministers. Others can apply for a hardship deferment, arguing that military service would place strain on their families, and some request conscientious objector status. A conscientious objector can fall into a number of categories, including someone who is opposed to all military service or someone who is opposed to carrying weapons, in which case he will be required to fulfill alternate service duties.