What Is State Income Tax?

Most states impose their own income tax on top of federal income tax, with rates and rules that vary widely. Some states follow federal rules closely; others have their own deductions, credits, and exemptions. You generally file a state return for any state where you earned income or lived during the year. Most state returns are due April 15, the same as federal returns. If you work remotely and travel, you may owe taxes in multiple states — even if you only lived in one.