Male US citizens (regardless of where they live) and male permanent resident aliens living in the US born after December 31, 1959, must register with Selective Service within 30 days of their 18th birthday (30 days before and after). If you fail to register during this period, you may submit a late registration until your 26th birthday.
Male non-citizens (including illegal aliens, legal permanent residents, seasonal agricultural workers, and refugees) who take up residency in the US before their 26th birthday are required to register. All relevant INS forms (e.g., the application for Resident Alien status, I-485, and so on) include a clear statement regarding the requirement to register.
Dual nationals of the US and another country must register regardless of where they live.
Conscientious objectors are required to register. If a draft is instituted, they can file a claim for exemption based on their religious or moral objections. But they must nevertheless register with Selective Service.
Disabled men who can move about independently in public with or without assistance must register with Selective Service, even if their disability would disqualify them from military service.
Reserve and National Guard members who are not on full-time active duty must register. Men attending the Merchant Marine Academy must register. Men who attempt to enlist and are rejected before age 26 must register.