Is dash cam illegal in US?

I'm curious to know, is it against the law to use a dash cam in the United States? I've heard some conflicting information about whether or not they're allowed, and I'm wondering if there are any specific laws or regulations surrounding their use. I understand that some states may have different rules, but is there a general consensus or guideline that applies nationwide? And what about privacy concerns - are there any legal implications to consider when recording footage with a dash cam?