Generally not required for stays of up to 90 days. Passport holders of EU member states can stay indefinitely.
Romania is part of the European Union and formalities for entering and leaving the country are much the same as for any other EU-member state.
Citizens of EU countries do not need visas to visit Romania and can stay indefinitely. Citizens of the USA, Canada, Australia, New Zealand, Israel, Japan and many other countries can stay for up to 90 days without a visa. Other nationalities should check current requirements with the Romanian embassy or consulate in their home country.
As visa requirements can change, check with the Ministry of Foreign Affairs (www.mae.ro) before departure.
All international visitors to Romania are required to have a valid passport (EU members need only a valid EU ID card). The expiration date of the passport should exceed your travel dates by at least three months, though some airlines will not allow passengers to board unless the passport is valid for at least six months.
Note that Romania, while a member of the EU, is not part of the EU’s common border and customs area known as the Schengen zone. In practice, this means that regardless of nationality you will have to show a passport or EU identity card when entering from EU member states Hungary and Bulgaria, as well as from non-EU states Ukraine, Moldova and Serbia.