What is the US requirement for providing proof of identity?

Paul gave a good comprehensive answer.
It depends on who wants proof?
Is it for a job?
A police officer who pulled you over?
Immigration and customs?

Thank you, let me state a specific. Do citizens owe law enforcement proof of identity whenever requested?

Thanks everyone. I am exploring this topic a bit in a place least likely to cause inflammatory discussion. I teach martial arts and wanted to explore this topic to more depth and appreciate your insights! :)

Thank you!

Hi John, I know the question has been closed, but I thought I would add something that may help your research.  Search YouTube for "Sovereign Citizen Gets Owned" (or similar).  In most cases law enforcement officers have just cause to have stopped a motorist and the "sovereign citizens" end up being detained in order that the law enforcers can establish their identities.

We have a situation in the UK where non-commercial drivers have no obligation to carry their driving licences at all times.  Provision is made for the cops to issue the driver with a form requiring them to produce the driving licence for inspection at a police office of the driver's own choosing within 7 days.  There is, however, provision in law that in certain circumstances (such as a vehicular collision) police may detain you for as long as is necessary to satisfy themselves of your identity if you do not have your driving licence.  Common sense tells you that, although you don't need to by law, it is in your best interest to carry your driving licence to avoid unnecessary complications that are quite legal.  It's a case of self-preservation, just like the principles of the martial arts that you teach.

Thanks Bill; as you surmise, I am trying to wrap my head around the topic of personal identity and what is legally versus reasonably expected of citizens. I love the pro/con you have setup by contrasting your experience in the UK. Great help; thank you!