Can a company in the US that developed a technology, and kept it secret without patenting it keep other companies from patenting the same technology?
<p>Let's say company A developed a technology, but didn't apply for a patent in the US.  Ten years later, Company B applies for a patent for the same technology. Can company A void the patent saying there's prior art that was kept secret by them?</p>

<p>No - prior art essentially means “<em>published</em> prior art”.  If a company decides to keep something a trade secret rather trying to patent (this involves disclosure) they take a chance that someone else will independently invent and  patent it. Under the 2012 AIA law in the US such a company can have limited rights to keep doing what they were doing, where they were doing it, to the extent they were doing it.</p>
<p>From Wikipedia</p>
<blockquote>
<p>Prior art (state of the art or background art), in most systems of patent law, is constituted by all information that has been made available to the public in any form before a given date that Prior art (state of the art or background art), in most systems of patent law, is constituted by all information that has been <em>made available to the public</em> in any form before a given date that might be relevant to a patent's claims of originality. be relevant to a patent's claims of originality.</p>
</blockquote>

united-states patents