What distinguishes manipulative interpersonal influence from non-manipulative influence? When is it wrong to manipulate a person and what makes it wrong? I articulate a novel account according to which interpersonal manipulation is a process of influence that deliberately fails to track reasons. To manipulate a reasons-responsive person is to render her detached from an important aspect of reality, namely, her reasons or the considerations that ought to govern her behavior. This is what makes manipulation pro tanto morally impermissible (when it is). My account of manipulation provides a helpful framework for thinking through some of the philosophically neglected ethical issues arising out the application of social scientific research on human decision making in the domains of health care and public policy.