An NDA is a contract that is a matter of state law, where a patent is a federal document, and therefore, interpreted uniformly throughout the country. If you sign a NDA with a company then they are obliged to honor it. If they take your idea and put it on the market, you have full capability of suing them. However, similar to what Viraj said, if a second company puts your idea on the market you can not sue anyone unless you have concrete evidence that the first company leaked the idea to them. However, if the same scenario happened with a patent instead of a NDA all of the rights to the idea belong to the original person. NDA’s are still important for several reasons. They help remind people that they signed a document to not reveal or accidentally spread another person’s idea. NDAs are often used before or a patent is filed or completed, and can provide further back up in a case where someone infringed a patent.