Italy specializes in bureaucracy. Here is an accounting of some of the most important documents that you might need in order to live in Italy in compliance with the law. Be sure to keep copies of everything. If you’ve moved to Italy, you might find a copier/scanner a useful investment.
Under current law, you can visit Italy as a United States citizen for up to 90 days without a visa. For longer stays, you will need a visa. Various types are available, including: work visas (very difficult to get, usually coordinated through your employer), student visas (course of study at least 20 hours/week, renewable for up to 2 years); and family reunification (with immediate family member already resident in Italy).
For most people who want to live long-term in Italy and don’t need to work, the best option will be an Elective Residency visa.
You will want to obtain a Codice Fiscale. It is necessary for pretty much any sort of financial transaction and including some online purchases.
When you arrive in Italy with the intent to reside here, you are required to apply for a Permesso di Soggiorno within 8 days. Once you receive your Permesso, you can register your residence in the local Comune and obtain a Carta d’Identità. Normally, your Permesso will expire after one or two years. After you have legally resided in Italy for 5 years (and satisfied a number of other requirements such as A2 level Italian) you can obtain a Permesso di Soggiorno di Lungo Periodo the main benefit of which is that it doesn’t have to be renewed so often.
Folks who are lucky enough to have Italian heritage may qualify for Italian citizenship by the law of Jus Sanguinis. If you qualify, you are and always have been an Italian citizen, but you need to apply to have that fact recognized. The application can be done either at an Italian consulate or at your local Comune, depending on where you live. There is a Facebook group with detailed information on this topic.
An Italian citizen can obviously live here without further documentation. However, his or her immediate family can live here as well. To do so, the family member must obtain a Carta di Soggiorno di Familiare di un Cittadino dell’Unione at the local Questura. The most important piece of documentation needed to obtain this permit is the certification of family relationship (e.g., marriage). Normally, this information is recorded either in a Comune or in AIRE, the registry of Italian citizens abroad. But if this is not the case, you’ll need a certified, apostilled, and translated certificate from the relevant authority.
Spouses of Italian citizens can gain Italian citizenship by the process of Jus Matrimonii. In addition to appropriate birth documentation, criminal background checks, and more paperwork, JM citizenship also requires a B1 level of Italian language. The process can take up to 4 years.
Anyone who has lived legally in Italy for 10 years can apply for citizenship by Naturalization. This also requires B1 level language skill and a ton of paperwork.