Compatibility summary
You will need a travel adapter in Greece; plug types used there differ from United States. Voltage differs by more than 20% — a voltage converter may be needed for some appliances.
Plug comparison
United States plug types: A B
Greece plug types: C F
There is no common plug type between United States (A B) and Greece (C F). You need a travel adapter.
Voltage comparison
United States: 120V, 60Hz
Greece: 230V, 50Hz
Voltage converter recommended (United States 120V vs Greece 230V).