Emotion data is dangerous because of how much it reveals about us and how deeply it is coveted by industry and government. This data—purporting to represent the emotional, psychological, or physical status of people by processing their expressions, movements, behaviour or other physical or mental characteristics—is irresistible for companies to use to their advantage. Having failed to recognize this danger, the law has left people open to the exploitative whims of industry and government who process emotion data. This chapter proposes a duty of emotion data loyalty to prohibit recipients of emotion data from acting in ways that conflict with the best interests of the trusting party. The idea behind loyalty is simple: organizations should not process emotion data or design technologies that conflict with the best interests of trusting parties. Our emotions should not be tools with which to manipulate us. Loyalty is the antidote to opportunism: it shifts the legal duty from self-serving to other-serving and has a morality broader than the profit-maximization of neoliberal capitalism. Though loyalty has deep roots in our law, there is more than just abstract ethics and notions of honour to the duty of emotion data loyalty. Loyalty compels firm legal duties and prohibitions that, when breached, give rise to legal liability on the grounds of conflicts of interest or duty.