Almost all male U.S. citizens, and male aliens living in the U.S., who are 18 through 25, are required to register with Selective Service. In a crisis requiring a draft, men would be called in sequence determined by random lottery number and year of birth. A chart (PDF) of who must register is available.
Hospitalized or Incarcerated Men
Young men in hospitals, mental institutions or prisons do not have to register while they are committed. However, they must register within 30 days after being released if they have not yet reached their 26th birthday.
Disabled men who live at home must register with Selective Service if they can reasonably leave their homes and move about independently. A friend or relative may help a disabled man fill out the registration form if he can't do it himself. Men with disabilities that would disqualify them from military service still must register with Selective Service. Selective Service does not presently have authority to classify men, so even men with obvious handicaps must register now, and if needed, classifications would be determined later.
Full-Time Military Exempted From Requirement
Young men serving in the military on full-time active duty do not have to register. Those attending the service academies do not have to register. However, if a young man leaves the military before turning 26, he must register.
National Guard & Reserves
Members of the Reserve and National Guard not on full-time active duty must register.
Men who would be classified as conscientious objectors if they were drafted must also register with Selective Service. If a draft begins and they are called, they would have the opportunity to file a claim for exemption from military service based upon their religious or moral objection to war.