Since October 26th, 1997 Italy agreed to the Treaty of Shengen, which provides the abolition of the controls at the domestic borders, and, therefore, the attestation of complete freedom to circulate in those territories of all the countries that signed the treaty.
The thirty countries that, at the moment, agree to the Treaty of Shengen are, in order of implementation: Belgium, France, Germany, Luxembourg, Netherlands, Monaco, Portugal, Spain, Italy, Austria, Greece, Denmark, Finland, Sweden, Iceland, Norway, Slovenia, Estonia, Latvia, Lithuania, Poland, Czech Republic, Slovakia, Hungary, Malta, Switzerland, Liechtenstein, Bulgaria, Romania and Cyprus.

Therefore, for those who come from the countries that agreed to the treaty and want to come to Italy, it is not necessary to show any document to the border (except for Bulgaria, Romania and Cyprus that have not yet implemented in practice all the technical features necessary to join the Schengen area, and therefore, provisionally, they still maintain border controls); although our advise is to take a valid identity document with you.

For those tourists who come from those countries which agreed to the European Union (currently composed of 27 countries), but there are not in Shengen area, it is necessary to show at the airport or at the border an identity document valid for expatriation.
Those countries are Ireland and United Kingdom.

For those citizens belonging to countries that don't apply the Treaty of Shengen or that don't belong to the European Union, it is necessary to show a valid passport.
For these foreign citizens the maximum period of time used to stay abroad for touristic reasons is three months. After this period there must be an authorization by foreign Italian consulates.
As far as the request for the issue of the residence permit for more than 90 days and for working reasons is concerned, it should be presented at the immigration office at the police head-quarters.

For some foreign countries, beside the passport, it is necessary to show at the border a consular visa, released by the Ministry of Foreign Affairs or by its qualified diplomatic consular offices network.

The citizens belonging to countries like Switzerland, Vatican City, San Marino, Monaco, Argentina, Australia, Japan, USA, Israel, etc. are not subject to duty of visa.

A complete list of all countries can be found at the Italian Ministry of Foreign Affairs' website at http://www.esteri.it/en.

Children that travel with their parents, and that don't own a personal identity document, must be included in one of his/her parents' passport.

Those tourists that own a driving licence released by one of the European Union countries can drive those vehicles for which their driving licence is valid on the Italian territory, without the duty of converting their driving licence in the Italian one.

Vice versa, those tourists who own a driving licence released by a non-member country of the European Union, can drive on the Italian territory those vehicles for which their driving licence is valid, but they should not have lived in Italy for more than a year.
The foreign driving licence (non-European) must be accompanied by the international driving licence (released by the foreign country that has released the driving licence), that is an official translation in Italian.
Further information can be found at the Ministry of Transport and Infrastructures' website at http://www.mit.gov.it/patente.htm