There Oughta Be a Law: When Does(n't) the U.S. Common Rule Apply?

Using mobile health (mHealth) research as an extended example, this article provides an overview of when the Common Rule "applies" to a variety of activities, what might be meant when one says that the Common Rule does or does not "apply," the extent to which these different meanings of "apply" matter, and, when the Common Rule does apply (however that term is defined), how it applies.