Federal tax law is what determines who may claim a child as a dependent on a federal income tax return. Even if a state court order allocates the ability to claim the child to a noncustodial parent, the noncustodial parent must comply with the federal tax law to claim the dependent. The noncustodial parent must attach to his or her return a copy of the release of claim to exemption by the custodial parent, either a Form 8332, Release/Revocation of Release of Claim to Exemption for Child by Custodial Parent or a substantially similar document.
Refer to Publication 504, Divorced or Separated Individuals for more information on the special rule for children of divorced or separated parents (or parents who live apart).