In addiction, impaired control over drug use raises questions about the capacity of addicted persons to consent to participate in research studies in which they are given their drug of addiction. We review the case for doing such research, and the arguments that addiction does, and does not, prevent addicted persons from consenting to such research. We argue for a more nuanced view that acknowledges that while in some situations addiction impairs decision-making capacity, it does not eliminate such capacity. We conclude with some suggestions for recruiting addicted subjects and designing experiments in ways to obtain free and informed consent.