You can unless you've signed something that prohibits you from doing so. There's no default assumption of non-disclosure or non-competition when you're taking

You can unless you've signed something that prohibits you from doing so. There's no default assumption of non-disclosure or non-competition when you're taking on a project for a company as a consultant, even if you are required to transfer copyright; most will force you to sign something to that effect, but if they don't, or they don't do it right, you're free to do whatever the hell you want with the information in your head. A lot of these agreements teeter on the edge of enforceability, as well, and if you aren't forced to sign a non-compete it's going to be tough in most cases for a company to prove that a clean-room reimplementation violates the non-disclosure agreement unless you really are relying on information; however, I'd suggest that you consult a lawyer before wading into these waters, and 99% of the time it's not even close to worth the trouble. You're a lot more at risk from a lawsuit (even if you win) than the company is, and you shouldn't be burning most bridges anyhow.