Passport & Visa requiremens

If you are an European (EU) tourist you will need just your ID or Passport. A Visa is not required unless you are planning to obtain a residence permit (permesso di soggiorno). In this case, you will have to register at any Questura (police station).
 
A visa is not required for USA and others extra-european tourists holding a valid passport unless you are planning to stay in Italy more than 90 days. If, after entering Italy, the tourist decides he would like to stay more than 90 days, you have to apply for an extension at any police station.

If you are planning to visit italy for study or work and you need to stay longer , you should check your current visa requirements with the nearest Italian Consulate before departure.

The regulations stated above also apply to San Marino and the Vatican City. Passports are valid for 3 months and required by all except the EU nationals of Austria, Belgium, Croatia, Finland, France, Germany, Greece, Liechtenstein, Luxembourg, Malta, Monaco, Netherlands, Portugal, San Marino, Slovenia, Spain, Sweden and Switzerland who just need a valid national ID card.
Travelers visiting just one Schengen country should apply to the Consulate of that country. 

Registration

You won't need to register with the police because this is usually attended to by the hotel you will stay with.
If not (for example if you are staying with friends or parents) you will have to register within 3 days at the nearest police station.  

Disclaimer: We've tried to make the info on Travelitaly.com accurate, but it is provided 'as is' and we accept no responsibility for any loss, injury or inconvenience sustained by anyone resulting from this information as the above lists are liable to change at short notice. You should always verify critical information (like visas, health and safety, customs and transportation) with the relevant authorities (Enquire at consulate or consular section at embassy) before your travel.