Italy belongs to the European Union and is an associated member state of the Schengen agreement hence all EU citizens or citizens of Norway, Iceland, Liechtenstein, San Marino, Switzerland, Monaco or the Vatican City, may enter the country without any additional paperwork, provided they have a valid national ID or passport.
Nationals of selected countries do not require a visa for visits up to 90 days. The Ministry of Foreign Affairs lists all the countries currently covered by the visa-waiver agreement (among these Australia, Brazil, Canada, Japan, and USA). All citizens of countries outside the Schengen area must have a passport valid for at least 3 months after departure date.
To find out if you require a visa please visit http://vistoperitalia.esteri.it/home/en or contact the Italian Consulate or Embassy in your country.