The U.S. Department of Education takes the position that --- barring poverty or extreme circumstances --- parents are responsible for paying for their child's college education until that child reaches 24 years of age. The government's guidelines provide educational funding to students only when their parents simply do not have enough money to pay tuition. It doesn't matter if your parent claims you as a dependent on his tax return or not. As far as the federal government is concerned, you are still a dependent student and your parents have the first obligation to pay for your schooling.
If your parents are divorced, the parent with whom you resided for the most time during the past year is the one who is required to complete your FAFSA. This is not necessarily the parent who has legal custody of you. It's the parent who gave you the most financial support during the last calendar year. Problems sometimes arise because if that parent has remarried, the federal government requires that her current spouse's financial information be included on a FAFSA, also.
Only students who meet the government's criteria for independence can file a FAFSA without their parents' cooperation. You automatically become an independent student the year you turn 24. Otherwise, you must meet at least one of the government's requirements for independent student status, changed in 1992. Generally, you must be an orphan or a ward of the court, a veteran of the United States military, married or be responsible for the majority of the support of a dependent child of your own. Schools have the right to override your dependency status and some institutions will do that in unusual circumstances, such as if your parents are deceased or their whereabouts are unknown. If your parents refuse to cooperate with a FAFSA, your first move should be to see the financial aid administrator at your school to find out if there is anything the institution can do to help.
The federal government amended the Higher Education Opportunity Act in 2008 to allow financial aid administrators to give students unsubsidized Stafford loans even if their parents do not complete a FAFSA, but you would need documentation that they are not supporting you and proof of their refusal. However, unlike federal funding, these loans require repayment. If you have special circumstances, you can also enter this into your FAFSA form yourself for consideration, but the government does not consider your parents' refusal or the fact that you're not living with them as special circumstances.