Will flagged the key federal fact sheet/resource. It's also worth checking state law, as they add additional requirements in some states. The hard part about valid unpaid internships is that, as a general matter, interns should not be displacing employees and there is an expectation that the company is devoting time and resources to "teaching" the intern. Stated another way, if the intern is "profitable" from the company's perspective, that is a risk factor.

It really depends... I'd say that if an intern can be a value-add to a company and provide a return on investment, then they should get paid no matter what. However, often times firms with CF problems or no CFs (e.g, new self-funded search fund), need an unpaid internship as it's helpful to have an extra hand. In addition, it provides the student/intern with valuable experience that can't be replicated in a school setting, so it can certainly be a resume booster as well, Unpaid internships can be a win-win, or not... really depends on the situation.