Europe covers a family of round-pin sockets — Type C, Type E and Type F — none of which accept an American flat-pin plug. You need an adapter, and you need to think about voltage.
The adapter
Look for a US to EU adapter (sometimes labelled "A/B to C/E/F"). It has two round pins on the wall side and accepts US two- and three-pin plugs on the other. It fits France, Germany, Spain, Portugal, Italy, Greece, the Netherlands, Belgium, Austria, Croatia, the Nordics and most of Eastern Europe.
It does not fit Britain, Ireland, Switzerland or Denmark — those need their own adapters.
The voltage
Europe is 220–240 V / 50 Hz; the US is 120 V / 60 Hz. For dual-voltage devices (100–240 V) this is a non-event. For single-voltage devices it is the difference between working and failing.
- Fine with an adapter: phones, laptops, tablets, cameras, e-readers, electric toothbrushes, most CPAP machines.
- Needs a converter: hairdryers, curling irons, travel kettles, some blenders.
- Check before you pack: anything with a motor or a heating element.
Frequency is mostly a non-issue
50 Hz versus 60 Hz affects synchronous motors — clocks, older fans, some turntables — but not the switch-mode power supplies in modern electronics. A US alarm clock will keep the wrong time in Europe; a laptop will not care.
Grounded plugs
US three-pin Type B plugs need an adapter with an earth connection. Cheap "travel adapters" often pass live and neutral but leave the earth unconnected, which turns an earthed appliance into an unearthed one. If you are travelling with a laptop that has a metal case, buy an adapter that specifies earthing.