When you prepare your federal income tax return, it's likely you want to seek every possible deduction to reduce the amount of tax you owe. One way to do this is by claiming your children and other qualifying relatives as dependents. Each dependent claimed lowers your total taxable income by one exemption.
Each dependent must meet age, relationship, residency and support tests. Additionally, each person can only be claimed as a dependent on one income tax return per year. So if you share custody with a former spouse, only one of you can claim the child.