Depending on local laws such as your country, state, district, or county:

Cars without license plates require Tags. These identifiers are used to show your vehicle was inspected and registered with the local authority such as the DMV. Because new plates take time to be obtained, Tags can be placed in either the windshield or on an unqualified plate.

There are usually some circumstances where a vehicle may drive without a plate, such as a federal authority or in an emergency.