Who Must Register

Almost every male U.S. citizen between the ages of 18 and 25 must register with the Selective Service System within 30 days of his 18th birthday, before or after. This includes cadets at the Merchant Marine Academy, ROTC students, people who left active duty armed forces before the age of 26, National Guardsmen and Reservists who are currently not on active duty and Civil Air Patrol members.
Some non-citizens are also required to register if living in the United States. These include resident aliens, seasonal agricultural workers, illegal aliens, refugees, asylum seekers and dual-nationals.
Women do not have to register because Selective Service law specifies "male persons." For women to be required to register with Selective Service, Congress would have to amend the law.
For a chart that also includes those exempt from registering, see resources.

What Happens if You Fail to Register

If a person who is legally required to register for Selective Service fails to do so, that person might be prosecuted. Penalties if found guilty include fines of up to $250,000 and five years in prison. Even if a person is not prosecuted for failing to register, that person can still be denied student aid, federal job training and most federal jobs.

Only Children

Only children, only sons and sole-surviving sons can still be drafted. Any siblings or children of a soldier who falls in combat can be awarded a peacetime deferment in case of a draft outside of a time of war, but can still be drafted in wartime.

How a Draft is Called

Drafts are setup during a time of national crisis, such as World War II or Vietnam, and are started by legislation passed by Congress and then signed by the President.
At this point, lottery numbers are assigned based on birthdays with males turning 20 within a given year being drafted first, followed by (if needed) 21-, 22-, 23-, 24- and 25-year-olds, in that order.

Lottery classifications

Once a draft begins, eligible males are classified into different categories.

1-A: available immediately for service
1-O: Conscientious objector who will do alternative service
1-A-O: Conscientious objector who will do non-combat service in the military
2-D: ministerial students. Deferred from service
3-A: Hardship deferments whose drafting would cause a hardship on his family
4-C: aliens or dual-nationals who are sometimes exempt
4-D: Religious ministers who are exempt