Preemption technically means situations where federal law displaces state law: a function of the supremacy clause of the US Constitution. Often, lawyers speak of preemption even where it is one federal law displacing another or one state law displacing another. When statutory laws abut or overlap like tectonic plates, which should apply?
As large-scale cases proliferate under federal and state wage-and-hour laws, there is more and more reason to study plate tectonics for potential defenses. Thinking about preemption requires looking beyond the intricacies of the case at hand to broader issues of public policy; applying preemption as a defense requires thinking about more than the statute alleged in the complaint.
Finding preemption, like throwing the Eephus pitch, is an arcane but game-winning skill. Learn how to find it in this article from Michael Giambona.