So I was accepted into a paid internship for this summer (May 19th - July 18) and I was wondering if I needed a working permit or visa for this? I will be located in Florence, Italy. I know I don't need a visa if I don't live there for more than 90 days, but if I want to get paid do I need both or either of these document?

What is your citizenship?

United States. I'm having a hard time finding the correct information and feel like my situation is different than what many say on most websites.

Ask your employer, they most likely have experience in the matter.

I think working in Italy without a work visa (very difficult to get and your employer would need to cooperate) is illegal for americans - unless it;s part of some approved study program.

However, I am not an expert.

The Italian government is the expert.

Contact the nearest consulate and ask them.

And why do you "feel your situation is different"?

I am a business owner who has american interns all the time - through study abroad programs / universities only and unpaid (they get credit for it). A paid internship, no matter what nationality you are, needs paperwork which is not possible if you are a US citizen and have no work permit. Ask the company how they are intending to pay you and what paperwork needs to be done. I am afraid the answer might be "cash & no paperwork", especially if it is a small company. If they are serious, you will need to start the paperwork yesterday.

The "cash and no paperwork" situation mentioned by vinoroma is rather risky, because if there's any sort of "misunderstanding" about your pay or work conditions, you have no recourse at all, given your illegal position. This is quite apart from the ethics of the situation.

You need to get this sorted BEFORE you fly over to Italy. If the border official suspects you may be working and you have no work visa (Schengen D-type), you can be refused entry and sent home.