If you work in Washington, D.C., Pennsylvania, Virginia and West Virginia
If you live in Maryland and work in Washington, D.C., Pennsylvania, Virginia or West Virginia you should file your state income tax return with Maryland.
If you commute to work in one of the states listed above, and your employer withholds taxes for that jurisdiction, you can file the appropriate form with that locality to obtain a refund.
If you work in any other state on a regular basis, it's a good idea to contact that state's taxing authority to determine your taxable status from their perspective.
If you have income that is taxed in another state, see Maryland Form 502CR to determine if you are eligible for a tax credit. Be sure to include your completed Form 502CR with your Maryland return, along with a copy of the return you filed with the other state.
If you work in Delaware
Maryland residents who work in Delaware must file tax returns with both states. To avoid dual taxation, you can get a credit for taxes paid to Delaware by completing Maryland Form 502CR and filing it with your Maryland income tax return. Be sure to include a copy of your Delaware return.