assuming your ex does not live with you and assuming you are the custodial parent (the one the child lives with more than 1/2 the year) and you meet all the other rules for claiming her, the tax laws say you can claim her every year. if you have a court order specifying what year each gets to claim her and it's his turn to claim, I recommend getting written approval from him to claim her.
if he's not the custodial parent in the year's he claims her, but you are, the IRS says you need to furnish him with a sign form 8332 granting him permission to claim her and he need s to attach that form to his filed 1040. without it he's not supposed to claim her.
