my ex wife has full custody of our son but he's been living with me the past 4 yrs now she says she claiming him on her taxes can she do that

Yes, and no.  Federal taxes, especially, don't have much to do with State Court Orders that formerly stated which parent can claim a child on their taxes.  If the two parents try to claim the same child, the second one who files will get a notice that they cannot claim the child since that child has already been claimed by someone else. The document will inform you what to do about it, but in the past, Court Orders and people (by Agreement) have decided who gets the tax exemption.  If your son has been living with you for 4 years but the last Court Order or Contract of Settlement between you and her gave her the exemption, then she has the right to do so.  Courts now do not award tax exemptions since divorce and custody are heard in State Courts and tax deductions/exemptions are part of the Federal system and stand on their own.  If parties agree on that issue, however, it will not be changed by the Court if you go back to Court for that purpose. If you go back to court to change custody, that exemption issue will follow the new Court's Order, but it will not be decided in Court.  It will be what the I.R.S. determines as the person with whom the child resided for more than 50% of the year.  So, if there is no Court Order or Agreement where your ex gets to claim your son, you can claim him but when and if the IRS gets involved, you should be ready to show them proof that he was residing with you - school records that give his address, etc.    Hope this helps, and Happy New Year. Danielle D. D'Eor-Hynes, Family Law Center, LLC www.hynesfamilylaw.com 478-971-1877